examples: aerospike example (#250)

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+#go-git + aerospike: a git repository backed by a database
+
+<img src="https://upload.wikimedia.org/wikipedia/en/2/2b/Aerospike_logo.png" align="right"/> This is an example of a [go-git](https://github.com/src-d/go-git) repository backed by [Aerospike](http://www.aerospike.com/). 
+
+
+
+
+### and what this means ...
+*git* has as very well defined storage system, the `.git` directory, present on any repository. This is the place where `git` stores al the [`objects`](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects), [`references`](https://git-scm.com/book/es/v2/Git-Internals-Git-References) and [`configuration`](https://git-scm.com/docs/git-config#_configuration_file). This information is stored in plain files.
+
+Our original **go-git** version was designed to work in memory, some time after we added support to read the `.git`, and now we have added support for fully customized  [storages](https://godoc.org/github.com/src-d/go-git#Storer).
+
+This means that the internal database of any repository can be saved and accessed on any support, databases, distributed filesystems, etc. This functionality is pretty similar to the [libgit2 backends](http://blog.deveo.com/your-git-repository-in-a-database-pluggable-backends-in-libgit2/)
+
+
+Installation
+------------
+
+What do you need? You need an *aerospike* server. The easiest way to get one for testing is running the official **docker** container provided by Aerospike:
+
+```
+docker run -d -p 3000:3000 --name aerospike aerospike/aerospike-server
+```
+
+Now, we need the sample code.
+
+```
+go get -u github.com/mcuadros/go-git-aerospike/...
+```
+
+Running this command will make the binary `go-git-aerospike`. if you have `GOPATH` on your `PATH`, you are ready to go. If not, this is a great moment.
+
+Usage
+-----
+
+### Cloning the repository into the database
+
+Running the command `go-git-aerospike` with the `clone` option followed by the URL of a git repository clones the repository into the database, storing all the git objects in it:
+
+```sh
+go-git-aerospike clone https://github.com/src-d/flamingo.git
+```
+
+The repository is stored in the aerospike database. This means that all the internal objects like commits, trees, blobs and tags are `records` in different `sets` in the `test` namespace:
+
+```sql
+aql> SELECT hash, type, url FROM test.commit
+```
+
+```
++--------------------------------------------+----------+-------+-----------------------------------+
+| hash                                       | type     | blob  | url                               |
++--------------------------------------------+----------+-------+-----------------------------------+
+| "c94450c805876e49b38d2ff1103b8c09cdd2aef4" | "commit" | 00 00 | ...github.com/src-d/flamingo.git" |
+| "7f71640877608ee9cfe584fac216f03f9aebb523" | "commit" | 00 00 | ...github.com/src-d/flamingo.git" |
+| "255f097450dd91812c4eb7b9e0d3a4f034f2acaf" | "commit" | 00 00 | ...github.com/src-d/flamingo.git" |
++--------------------------------------------+----------+-------+-----------------------------------+
+102 rows in set (0.071 secs)
+```
+
+And also the references and the configuration (remotes) are stored in it.
+
+```sql
+aql> SELECT name, target, url FROM test.reference
+```
+```
++------------------------------+--------------------------------------------+-----------------------+
+| name                         | target                                     | url                   |
++------------------------------+--------------------------------------------+-----------------------+
+| "HEAD"                       | "ref: refs/heads/master"                   | ...rc-d/flamingo.git" |
+| "refs/heads/master"          | "ed3e1aa2e46584cb803ed356cb5d8855f6d05660" | ...rc-d/flamingo.git" |
+| "refs/remotes/origin/master" | "ed3e1aa2e46584cb803ed356cb5d8855f6d05660" | ...rc-d/flamingo.git" |
++------------------------------+--------------------------------------------+-----------------------+
+3 rows in set (0.046 secs)
+```
+
+### Reading the repository
+
+Running the `log` command, a `git log --online` like result is printed:
+
+```sh
+go-git-aerospike log https://github.com/src-d/flamingo.git
+```
+
+The URL of the repository is the way we identify the objects in the `set`s, since we can clone several repositories to the same database.
+
+```
+ed3e1aa ID is also allowed in SendFormTo and SayTo
+2031f3e Handle close of message channel in WaitForMessage
+e784495 Add SendFormTo and SayTo
+447748a Make text in attachments accept markdown
+595b4e7 Form author name and author icon and text groupfield
+0f2e315 Test for InvokeAction
+0dc7c9a Handle closing of channel
+b3f167b Implement InvokeAction
+```
+
+The process has read all the commits and all the needed objects from the aerospike sets.
+
+### Playing with the database
+
+If you want to explore the database, you can execute the `aql` tool and run some queries:
+
+```sh
+docker run -it aerospike/aerospike-tools aql -h 172.17.0.1
+aql> SELECT * FROM test;
+```