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

Change-Id: I1af5659e782a272910c4499d7992cc6cc5ae3bcd
Merge "Gson deserialisation and refactoring"