Update to make dancer.js work again in Firefox.

Basically:
- rename the webkit backend to webaudio (as web audio is supported in Gecko, now), update callsites and paths ;
- Fuzz a clock test a bit more to make it green ;
- Alter the readme to reflect the changes ; 
- Update the change log ;
- Make sure use the new Web Audio names (the other are deprecated), with proper fallback ;