Update skulpt to python 2.7.6

@bnmnetp You mentioned something about how dictionary comprehensions would be cool :smile:

- Updates grammar and ast to 2.7.6.
- implements set and dictionary comprehensions.

@rixner and brad, all test pass. There are new objects for `repr` and `set` in the ast model. Those aren't used yet. I don't have more time to dive into that. Do you know of any other new syntax things in 2.7.6. 

I've also been thinking about adding a `debugger` statement (or a builtin) that adds a javascript debugger  statement to the code. A statement would be best because the browser debugger would stop in the __generated__ code.