Merge pull request #30 from mcamou/master

Add a sanity check so EmbeddableValues can only be mixed into BaseEntiti...
DomainObject is a layer supertype for BaseEntity
Moved 'version' attr back to BaseEntity: do not try to define metaprogrammed attrs twice in LTI
Unregister an object of a transaction
Merge pull request #32 from javierabstra/master

Unregister an object from a transaction