Support traceur script.

This pull adds support for google traceur compiler [[1]], to enable compilation of "JavaScript.next-to-JavaScript-of-today".

I'm sorry about the looks of the body of `compileString()`, but this is the state of things if you'd want to compile a string of traceur to js as of today. 

I added a couple of tests along with the coffee script ones.

Best regards,
Torgeir

[1]: https://github.com/google/traceur-compiler