Add comments to parser to indicate where ES6 features are parsed.

Another piece of [issue 499](https://code.google.com/p/esprima/issues/detail?id=499). I was curious to learn more about Esprima's ES6 support, and I thought it would be helpful to add comments in the parser to indicate where various features are parsed. All comments go to the ECMAScript wiki, but I could also update them to refer to sections in the draft specification if you think that would be better.