adds web audio builder from waveform-data.js

Adds web audio builder support (See: https://github.com/bbcrd/waveform-data.js/pull/18)

Note: Changed bower to link to my public fork of waveform-data. Will need to change back to BBC ref, when that is merged in.
This will not be needed anymore as the WebAudio builder is now directly attached to `WaveformData.builders.webaudio`.