Fix test for References
Added the BaseEntity#immutable method to return an immutable copy of the BaseEntity
Renamed ReferenceEntity to LazyEntityReference
Fixed 001_simple_entity_spec test
Added implementation of Class Table Inheritance
Refactor add_attribute to call add_attribute_accessors, which now only takes the descriptor
More Class Table Inheritance tests
Added immutable references from one root to another
Fixed a test
Fixes to Immutable, CTI and a few TODOs after code review
Merge pull request #15 from mcamou/master

Added Class Table Inheritance and Immutable References