Added some SDL_Mixer calls:
- `Mix_Playing`
- `Mix_Pause`
- `Mix_Pause`
- `Mix_Resume`

Includes test that tests the basic functionality of SDL_Mixer including pausing and resuming both the Sound channels and Music channel.
Made sure to set the `paused` property when controlling channel playback.
Merge branch 'sdl-mix-test' of https://github.com/michaeljbishop/emscripten into sdl-mix-test