Adds Audio::Music to hold music data.

Engine::Audio now plays the music it is given.

Next: To handle looping and callbacks for music finishing.

Afterward: To handle system events in a manner that can be used in
objects. For instance, the music ending should trigger an Event that can
be placed on an object! That'd be cool at least.
Adds Audio::Sound.

Fixes bug in SDL_mixer to deallocate wave data correctly.

Next: Need to manage the audio channels in Engine::Audio.