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.

 
Ideally there'd be more code sharing between the lambda path and the functiondef path, since this is missing a couple features that normal functiondefs would have (closures, memoization of the created CLFunction object).  Maybe `doFunctionDef()` and `_wrapFunction()` could be made to handle lambdas as well?