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 think we should expose this type to the SDK user. If we make it a concrete type in the SDK's API we can add other metainformation returned by the HTTP API in the future. My vote is to return this type as `AchievementList` and have it implement whatever enumeration protocol is most appropriate in Android code.

:smiley: