[3.1] Typings

[mikerockett]

Whilst this hasn't been written in Typescript, it's probably a good idea to add typings for better IDE support. tsc can do it through source and jsdoc introspection, so that's probably the route I'll take.

That being said, I'll need to become acquainted with the type system a little – not entirely sure what all the types are and how they can be used in jsdocs.