setting the timeout value on the client socket

this prevents gevent's sendall method from exploding when the
response is big enough to take multiple calls to send.

found that the proxy would choke when attempting to stream a large (255kb) http response to a laggy(ish) client. 

```
2014-01-31 20:37:30 [24996] [DEBUG] exiting weirdify True
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/gevent/greenlet.py", line 327, in run
    result = self._run(*self.args, **self.kwargs)
  File "/home/gadavis/vaurien/vaurien/proxy.py", line 215, in _weirdify
    behavior, behavior_name)
  File "/home/gadavis/vaurien/vaurien/proxy.py", line 163, in _weirdify
    return self.handler(source, dest, to_backend, behavior)
  File "/home/gadavis/vaurien/vaurien/protocols/base.py", line 58, in __call__
    return self._handle(source, dest, to_backend)
  File "/home/gadavis/vaurien/vaurien/protocols/http.py", line 62, in _handle
    source.sendall(data)
  File "/usr/local/lib/python2.7/site-packages/gevent/socket.py", line 471, in sendall
    raise timeout('timed out')
```

after adding some debugging to `socket.py` I found that the timeout value is in fact zero, hence the patch.

I haven't been able to work out a suitable automatic test case as yet.