Refactoring: Adding preference manager
 - new PreferenceManager was added with static methods that enable the
store and retreival of specific preferences needed in the code
 - MainActivity was refactored to use our preference manager when
checking the notification service and on analysing users gestures
 - WsDataLoader was refactored to use our preference manager when
storing the sync timestamp
 - NotificationService was refactored to use our preference manager when
storing or retreiving sync timestamp and when storing or retreiving user
GCM registration id.