JavaScript code hinting

As described on [Trello card 775](https://trello.com/card/2-code-hinting-javascript/4f90a6d98f77505d7940ce88/775). 

Commit history and other work in progress is in the [original repository](https://github.com/iwehrman/brackets-js-code-hints/). 

A copy of [Esprima](http://esprima.org), originally tagged as version 1.0.2, is in the local `thirdparty/` subdirectory. 

This extension was co-authored with @joelrbrandt, but I deserve all of the blame for the bugs :)