add createMediaStreamDestination

methods and interface are defined in http://webaudio.github.io/web-audio-api/ and implemented in latest chrome versions

I'm not sure to really undertsand how the directory is organized with waa.d.ts, waa-nightly.d.ts and waa-2012-08-02.d.ts, my understanding was that this should only go to waa.d.ts (according to the specs referenced in headers), and also not in test file.