Introduce TypeScript & ES6

[piotrl]

I don't plan completely rewrite nor introducing ES6 classes (which sucks).

But introducing types will help reading codebase (which is not big though, but still)

ES6 will help to get rid of promise-like utils that I'm using right now.

• [x] Create interfaces of API
• [ ] Remove promise-like utils

Cons:

• Will need to compile code to ES5 with polyfills, file will be bigger..