Decouple entities from keyspace in driver-mapping.

The keyspace parameter of the Table annotation is now optional. If an entity does not specify a keyspace
in Table, the keyspace from the MappingManager's Session will be used.

(JAVA-442)
Fix bugs in @Frozen checks (JAVA-441).

* include description of cause in error message
* don't expect enums to be frozen
Merge pull request #222 from dananderson/2.1.