Convert module to TypeScript

[jswalden]

I grumped in #33 about how I was adding jsdoc @type tags all over in an attempt to have poor-man's typing of all my changes, but really the code would be better served by actual types enforced by an actual compiler.

Now that I've been given maintainer access, and the previous maintainer has pretty much disclaimed knowledge/interest here, I get to make my wish reality. :-) I don't think TypeScript is so terribly different from JavaScript as to be a huge burden on anyone proposing patches, and the typing benefits are worth a little inconvenience.

These changes are atop #36 and will, like that change, not be part of the 3.0.0 release but of a future release.