Add TypeScript definitions

[secretfader]

I'm writing an app in TypeScript, and it would be nice to have definitions to aid in syntax completion, etc.

Provided that you're open to this, I'll finish what I've written so far and submit a pull request.

[jeffbski]

Absolutely. I knew that we needed it, but I'm not versed enough in TS to do it yet. So any help is appreciated.

[rob3c]

@nicholaswyoung Did you ever end up distributing your type definitions somewhere?

[jeffbski]

added in 0.15.3