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.
From http://tools.android.com/tech-docs/new-build-system
```
* DSL/API changes:
    * Renamed a few properties to make things more consistent.
        * BuildType.runProguard                 ->  minifyEnabled
```