Implemented music manager

It's also a singleton class, very similar to the image manager.

Added functions to load, play, toggle, and free music.

Also changed the image manager constructor to correctly intialize image
support. Also quits both SDL_mixer and SDL_image.

Also changed the image manager (and now music manager) to include base
paths for the filepaths where images and music are loaded from. This is
subject to change.

Plan on implementing similar class to manage sound effects (similar to
current image and music managers in that they are singletons, load by
id, etc); it's an effective way to manage all this media/content.