Fix deprecated web-audio api:
GainNode.gain.value instead of AudioBufferSourceNode.gain.
AudioBufferSourceNode.start() instead of AudioBufferSourceNode.noteGrainOn().
AudioBufferSourceNode.stop() instead of AudioBufferSourceNode.noteOff().
Fix web-audio api checking. Use window.AudioContext instead of window.webkitAudioContext
Fix for lower compatibility of webaudio api.
Merge pull request #234 from kumabook/fix-webaudio

Fix webaudio