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.
Thinking about it, this value is identical to the  `mediaElement.src` right?

So instead of making an extra request, there might be a use case for `AudioContext.createMediaElementSource()` and read the binary data from the element on `load` (job done) or `receiveddata` (partial rendering, to be done).