Add test cases and function to get a class name from an XML Object Type
Create classes for most the main core types.

Observables, Observable, StatefulMeasure, Object, DefinedObject.

I think there's still work to do combining the Object and DefinedObject layers.
Namespace cleanup.

- Observables, Observable, StatefulMeasure, and Object classes should be
  importable from cybox.core instead of their respective files.
- Removed DefinedObject from core (already exists in common).
- Merged Observables and Observable into same file.