MOBSDK-480/481/486

MOBSDK-480/481:
- added a new class and protocol to utils. AlfrescoCache (protocol for other data we might want to cache) and its implementation AlfrescoSiteCache. 
- added new methods for add/remove fav., join/leave/cancel site as of spec
- added 3 new unit tests that test join/cancel (moderated site), join/leave (public site), add/remove fav site

MOBSDK-486: modelClassVersion is a number added to each model (except content file - see comment). The version number is defined in the internalconstants file