Initial SoundSprite Implementation

Known issue with AudioElement when playing multiple sounds at once.

Please see http://www.stagexl.org/forum.html?place=msg%2Fstagexl%2F4RS-FFPh4u8%2Fq1wHEQiDyMcJ for explanation.

Demo http://realbluesky.github.io/DartPiano/build/web/