Implement lambda expressions

I have updated my old https://github.com/dropbox/pyston/pull/77 pull request to base against the current head and changed a few things slightly.
I apologize for creating a new pull request but because of my lacking git skills I had to.

Please review it again and tell me what I have to change.

 
I don't think this will hold if we have a lambda inside a function.  I think it just needs to be implemented the same way as visit_functiondef?