Added Sound.release() and Assets.getMusic().

Sound.release allows you to free up audio resources immediately rather than
waiting for GC, which can be fiddly if you're up against the wall.
Assets.getMusic() allows us to differentiate between short SFX clips
(getSound()) and longer, possibly streamed music (getMusic()).

A forthcoming patch to the Android backend will make use of this, but I will
probably also change the Java backend to use BigClip only on getMusic() calls.
A new sound implementation based on SoundPool.

This is used for getSound() and the old MediaPlayer-per-sound implementation is
used for getMusic(). SoundPool is way more resource efficient for playing short
SFX but can't handle music, so it behooves us to differentiate this at the
PlayN API level, which we now do.