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.
What do you think about making this directly support the `Iterable` interface? and renaming it `AchievementList`?