Achievement support and Server endpoint support

* Added support for `/v0/server` endpoint.
* Added support for `/v0/game/achievement` endpoint.
* Added support for `/v0/gamer/achievement` endpoint.
* Upgraded to OkHttp 2.1.0, Okio 1.0.1 and Gson 2.3.1.
* Now requires Android Build Tools 21.1.1 and Android Gradle plugin 0.14.4.
I'm trying to keep the test code pure Android, to serve as a vanilla example if needed. I agree it's a bit extreme to apply that to data classes like this, though. I can drop in Lombok for this, if you like?