PoC to swap esprima with Traceur

Hello.

This is a PoC changeset demonstrating a fairly successful attempt at swapping esprima out for the traceur-compiler parser.

Why would you want to do this? Well, Traceur supports basically all ES6 syntax *today*. Currently, if you run eslint over an ES6 source file, you'll get lots of errors if you're using advanced ES6 syntax (classes / modules / etc).

So what is this changeset doing? It uses Traceur to parse the source file. Then, using my [traceur-mozilla-ast](https://bitbucket.org/atlassianlabs/traceur-mozilla-ast) library, it converts the Traceur-specific AST and tokens into Spidermonkey/esprima AST equivalents. As a result, the changes required to the codebase are fairly minimal. Most of them focus on the `parse` method in `lib/eslint.js`.

The result is that a vast majority of the tests pass. Self-validating the eslint codebase with itself passes without any errors. There's 16 failures currently, with a bunch of them specific to sequence expressions and the use of the `let` keyword. I haven't tracked these down yet, purely because I wanted to see what you thought of this idea before I worked on it any further :)

If you think this is worth pursuing further, I could see it being properly implemented by allowing Traceur / esprima to be chosen as a commandline/API option. This way, eslint users who are starting to incorporate ES6 features into their codebase can opt into the Traceur compilation, but existing users won't be risking a potentially unstable change of underlying parser ;)