#24650 Added bundle.execute_hook_method() and sgtk.get_hook_baseclass()

Adds a new method execute_hook_method() to bundle (e.g.
engine/app/framwork) objects. This allows app code to execute methods
beyond the standard execute() method that execute_hook() executes, and
allows you to structure hooks more like classes, where one hook can
contain a number of different methods. Furthermore, there is now a 
proxy method sgtk.get_hook_baseclass() which forms the foundation for
the upcoming hook inheritance support.

This squashed merge closes #58