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.
This is currently not returned to the SDK consumer. If we intend to do that, then yes, I like this idea.