ES6 rewrite

[thomas-alrek]

The engine should be rewritten in ES6 (ES2015).
Then we don't have to implement our own extend and clone methods

[arrterian]

Hello, we want rewrite your project to ES6 together with student of our JavaScript courses for better understanding differences between ES6 and ES5.

[thomas-alrek]

@RomanValihura yeah, sure great. Just submit a pull request, and I will review it