Add typescript definition for public API.

[chriseppstein]

Hi, I use this project with a project written in typescript so I made a typescript definition for it.

If you're not familiar with typescript there's two approaches to adding a public definition file. The preferred approach is to add it to the project and ship with it. This way new changes to the API can be kept in sync. The second approach is that this definition can be shipped as a separate node module and maintained by the typescript community at https://github.com/DefinitelyTyped/DefinitelyTyped

If you're not interested in accepting this PR then please let me know and I'll take the community driven approach instead. If you are, and would like help maintaining the typescript definition when the public API changes, I'm willing to do so.

[coveralls]

Coverage increased (+0.3%) to 98.904% when pulling 3deaac1 on chriseppstein:typescript into 018ae87 on fb55:master.

[chriseppstein]

@fb55 I won't have time to work on a full port to typescript for a few weeks to a month can we land the definition file for now?

[fb55]

Sure thing :)

[chriseppstein]

@fb55 Thanks! will it be released to npm in the near future?