tcp proxy

Main PR: https://github.com/mitmproxy/mitmproxy/pull/209

- Add `tcp.Address` to unify address handling
- Change `netlib.wsgi` Request class to match the signature of HTTPRequest in mitmproxy@tcp_proxy
- Minor refactorings (combine `BaseHandler/TCPClient` `.close()` and `.finish()` methods in SocketCloseMixin)