Gson deserialisation and refactoring

GerritService uses a more generalised SyncProcessor class to help with handling the different requests
The projects list is now deserialised by the Volley GsonRequest class
Added gson library to build.gradle
Added Project custom json deserialization
Added Reviewer (committer) custom json deserialization
Fix some differences from the master branch

Change-Id: I1af5659e782a272910c4499d7992cc6cc5ae3bcd