Changes towards to the standard Web Audio API

This pull-request:
- Makes retro-audio-js compatible with Firefox Nightly
- Uses the standard WebAudio API methods
- Was tested on Firefox Nightly 26.0a1 (2013-09-15) on Ubuntu 13.04
- Was tested on Chromiun Version 28.0.1500.71 on Ubuntu 13.04
- Was tested on Google Chrome Version 31.0.1626.0 dev on Ubuntu 13.04