better cross-browser audio recording

Chrome works, but is fairly quite (question about that filed over at https://www.webrtc-experiment.com/RecordRTC). Not tested against all mobile devices yet.

* [x] firefox desktop
* [x] Chrome desktop (albeit fairly quiet)
* [x] FF for android
* [ ] Chrome on android fails (we don't care?)
* [ ] FFOS (no device available at home)