Reduce the number of libs used

[pedrovgs]

As we already know, the usage of third party libraries should be limited when developing a library. That's why we should get rid of OkHttp and Gson in the future once the MVP be released. This should be one of the first tasks in the next iteration. Right @davideme ?

[pedrovgs]

We could use jar jar to repackage google play services lib :)

[pedrovgs]

The branch named "try-to-replace-dependencies-using-jarjar" contains a first approach using the Jarjar gradle plugin: https://github.com/vRallev/jarjar-gradle

[pedrovgs]

@flipper83 do you know if we can repackage an AAR? We can use this gradle plugin with jars but it does not work with aars :S