Merge pull request #33 from mcamou/master

to_hash now only serializes attributes defined in the BaseEntity instead of all instance variables;  to_nested_hash leaves LazyEntityReferences unresolved; Equality for LazyEntityReference and Version
TransactionRegistry returns a TransactionNotFound instead of nil
Better exception PessimisticTransaction handling
Transaction exceptions give object class and id information
Fixed module mismatch in exception
PessimisticTransaction's load_as_* not calling the public API