Feature/performance caching

Hi

Enclosed code contribution for caching feature for improving the REST API GET operations on a collection (gGRC objects and stub). 

The request patterns for GET includes:
  api/<resource>?id__in= 
  api/<resource>?__stubs__only=true

The caching mechanism used is Google AppEngine Memcache. 
The code also has SQLAlchemy event listeners at session for gGRC object changes as well as handling network errors in commits to avoid cache inconsistency with DB.

Regards
Mouli