Create type definitions (.d.ts) for Typescript

[mustafarian]

currently it's a little tricky to use this package in a typescript project.

To unroll in a Typescript project one needs to:

1. Install the package using npm.
2. Declare the module in typings.d.ts.
   declare module 'unroll'
3. Import it in the spec file like import * as unroll from 'unroll';

Ideally the second step should be avoided.