Refine register/unregister function

With this update, we can add/remove custom properties more easily.

`bpy.utils.register_class()` is not necessary because `bpy.utils.register_module()` will do that for us.
You can pass `verbose=True` like `bpy.utils.register_module(__name__, verbose=True)` to see some details.
(Reference: [Blender Artist Forums](http://blenderartists.org/forum/showthread.php?336867))

Hope this helps.