Add .d.ts file for TypeScript/IDE support

Created by: Hoishin

What is the current behavior? Miss the type definition file.

What is the expected behavior? Have the type definition file.

What is the motivation / use case for changing the behavior? Supports TypeScript and makes it easier to code in IDE.

I've already written the type definition from API documents in README. I'd be willing to open pull request here or at https://github.com/DefinitelyTyped/DefinitelyTyped. I'd like someone to review it though.