Simpler objects

This will hopefully make further development much easier, by reducing repetitive code between all of the objects, and allowing more generic stuff to be done in `cybox.Entity` class. Not all classes have been converted to using `TypedField`s (and I'm sure there are some hiccups that will prevent that from ever being done completely), but that should detract from any class-specific functionality, which can still be done per-class if the generic implementation in `cybox.Entity` is insufficient.