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.

 
Hmm I'm not sure this would work, since I think you will have cleared the original array?  Maybe we can just put an `assert(node->args->defaults.size() == 0)` here and punt on the defaults stuff for this commit.  Looks like the functiondef part gets this wrong too.