Proxy support

Add proxy support so the code which needs to be tested can use the server in proxy mode.
It means that a slightly different requests needs to be handled, in addition to the CONNECT method which is used for TLS.

Unfortunatelly werkzeug has no proxy support, so it may be better to use the wsgiprox library (https://github.com/webrecorder/wsgiprox). It also support TLS.
