Add a proxier that does what the bouncer does, but proxies requests instead of bouncing them.

The current bouncer tests are more end-to-end tests. We should keep these but also add tests for the individual bouncer request handler methods.

See #1 for the initial bouncer work.
That's weird [`_execute_handler()`](https://github.com/fiorix/cyclone/blob/master/cyclone/web.py#L1138) should call it though. Is this perhaps a cyclone version issue?