Added proxy support, but no tests

This PR adds simple proxy support to requests. Example usage:

```
def main(reactor, *args):
    d = treq.get('http://httpbin.org/get', proxy=('localhost', 8080))
    d.addCallback(print_response)
    return d

react(main, [])

```

I didn't add any tests because I couldn't find a logical place to put them. 

Thoughts?