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.
I think [`TestCase.addCleanup`](http://twistedmatrix.com/documents/current/api/twisted.trial.unittest.TestCase.html#addCleanup) gives you this?