Feature/add typescript type definitions

[0vidiu]

Why:
(Hi!) We've been using your wonderful module in a Front-End project that uses Typescript and to make things clearer for the devs (IDE intellisense) we wrote custom declaration files for it and included in the project itself. Although that works perfectly for us, we thought it might benefit others too, hence this PR.

How:
Given the fact that the module itself is written in JavaScript, we cannot automatically generate the declaration file. This means we wrote it manually by cross-referencing documentation and source code. Any updates to the module should be reflected in the declaration files as well to keep them up to date.

Need help:
Although I am comfortable writing Typescript, I'm by no means an expert at it, so I invite the community to improve my work.
One idea for the future would be to extend the Config interface by making it a generic interface, therefore Typescript could infer the return type of the fixtures method based on the request method.

Screenshots:

[pkuczynski]

@Slashgear @DevSide any chance to get this merged and released?