Fixed error where SDL_Mixer would run out of channels because sound
instances weren't removed when they were finished playing.
Added test to make sure channel allocation failed when it
was exhausted.

To run it:

    python tests/runner.py browser.test_sdl_audio_mix_channels
Removed unnecessary "audio.paused = " statements.
Merge pull request #1317 from michaeljbishop/sdl-mixer-sound-leak

SDL mixer sound leak