Introduce native TypeScript support

[robbieaverill]

We have TypeScript support now using the third-party DefinitelyTyped types repository, see #46.

We haven't really ironed out how we will handle type definition changes where they change in our API, given that our API versioning is date based and Commerce.js is currently using semantic versioning. One idea here is to switch to date based versioning in Commerce.js as well.

In Commerce.js v3 we would like TypeScript to be natively supported, which would allow us greater flexibility to couple Chec API and Commerce.js versions, and control over when and how the types need to change without third-party DT maintainer approval.