Incomplete - task : implement pyMongo repository 

Implemented NoSql repository intitialization.
Implementation of MongoDB filter expressions.
Made the MongoDB backend operational.
Worked on test coverage.
Added a representer configuration context; added an "ID" attribute to
linked resources.
Simplified test app ZCML configuration files
Unit test fixes.
Fixed a serious problem with loading entities from a file system
repository.
Brought coverage up to 100%.