Changed some functions to classmethods and staticmethods.

create_object makes more sense as a classmethod for StellarClass,
so it has been changed to one called StellarClass.create. So, for
example, you now would call ``MyClass.create(arg1, arg2)`` instead
of ``sge.create_object(MyClass, arg1, arg2)``, which is more clear.

sound_stop_all, music_clear_queue, and music_stop_all are a bit
less clear, but I think it makes sense for them to be static methods
since their names are all prefixed with "sound" or "music". These
functions have been changed to the static methods Sound.stop_all,
Music.clear_queue, and Music.stop_all, so you now call e.g.
``sge.Sound.stop_all()`` instead of ``sge.sound_stop_all()``.