Update build file with new dependencies
Update build file with new dependencies

* Use retrofit 1.9.0 and OkHttp 2.2.0
* Define them as transitive dependencies
* Add gradle task to build a jar with all
  dependencies included

kaaes/spotify-web-api-android,mattiamaestrini/spotify-web-api-android,sregg/spotify-web-api-android,rberidon/spotify-web-api-android,sregg/spotify-web-api-android,jitpack-io/spotify-web-api-android,mattiamaestrini/spotify-web-api-android,kaaes/spotify-web-api-android,jitpack-io/spotify-web-api-android,rberidon/spotify-web-api-android