Test exposing null-reference exceptions caused by mapped interface types for table-per-type hierarchies
Test showing the same situation but with classes that passes
If no base type is discernable (or can't be pulled from the Extends<T>() option), then don't set a default key mapping
Flesh out the tests to verify that the expected subclasses exist in the generated mapping
Treat subclasses with interface types the same as parent classes with interface types

- All tests passing
Merge pull request #211 from tgmayfield/SubclassInterfaces

Subclass interfaces. Fixes NullReferenceExceptions caused by a null type.BaseType when the subclass type is an interface.