Support ES6

[thosakwe]

I'd love to send a PR (or multiple) to get add ES6 parsing support to this package. I just need your approval.

[asgerf]

I would be happy to merge in a good quality, full-featured ES6 parser. 👍

If you want feedback on a partial ES6 parser (e.g. a PR that just adds let) please target a branch called es6. For merging into master and updating the pub package I would prefer to wait until there is a complete implementation.

Note that the test-262 folder should be updated to ES6.