Added TypeScript version of pen.js

[richardpawson]

Makes it easier to use the Pen functionality from within a TypeScript app.

[richardpawson]

Thanks for the Turtle Wax Pen. I wanted to use it in a TypeScript app and ended up converting the whole of pen.js to TypeScript, which was very easy. It makes Pen a whole lot nice to use as well, because of the type safety.