added checks for supporting the deprecated Web Audio API spec that is still used in some browsers. According to the current spec noteOn/noteOff is replaced by start/stop. Furthermore createGainNode is replaced by createGain. Also added support for AudioContext besides the prefixed webkitAudioContext. The former is currently only used in Firefox nightly builds and might be tricky as FF does not support mp3 files on all platforms, which we are using right now.