Better commonJs module integration

[Igmat]

This small fix allows better integration with commonJS module system and reflects it in TS definition.
Initially I've started to hacking it because of issue similar to #383. Workaround that proposed there breaks typings system. Unfortunately there is no small fix for types when project is compiled using webpack (and angular CLI) that provides good programming experience.

This fix also will provide better usage for ES6+ users, because it allows next syntax:

import { Resumable } from 'resumablejs';

Which is valid in both ES6+ and TS.

NOTES:

1. We could probably also add default import for using it as import Resumable from 'resumablejs';
2. It also will be great if you won't exclude d.ts files when publish this package to npm, so TS users won't have to explicitly add additional dependency for typings. In this case typings would be always up-to-date.

[saberone]

Pretty please with sugar on top. Can this be merged. I currently have a dependency on the github reop of @Igmat (props for the fix), and it seems to work fine.

[steffentchr]

Sounds good, sorry for missing in the first go-around.