added bower.json file

Bower needs this to register Tone.js as a bower package. I also registered it to my fork (http://bower.io/search/?q=tone.js) but it would make more sense if you registered it again yourself so it points to the main repo. Let me know when you've done it and I'll delete my inital registration.

http://bower.io/docs/creating-packages/

P.S. I'm absolutely loving this framework, well done!