Proxy server

Hey @davemo -- I think this is working well enough to push out for a 0.3.0 release. Could you review and hammer on this a bit?

The only thing I think it's missing is there's no opportunity for lineman users to specify custom app.get & app.post endpoints in express (since it goes straight from static to proxy). I think I'm content on waiting to add that feature until later, though.

Any other thoughts, @kbaribeau or @timwingfield?