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.
still not working: https://gist.github.com/justinvdm/f47104245528ab32ccac (neither is `default`)