Absolutely relative proxying

Depends on PR https://github.com/mitmproxy/mitmproxy/pull/233
Isolated Diff View: https://github.com/mitmproxy/mitmproxy/compare/untangle_circular_dependencies...absolutely_relative_proxying

Changes:
- Renamed reverse proxy command line switch from -P to -R
- Add arguments for manually specifying the expected http form (`--http-form-(in|out) (absolute|relative)` and the upstream server. Add warning that usually -R, -T and -F should be used.
- Added logic to make sure that -R, -T, -F and --upstream-server are mutually exclusive
- Add SSL-terminating capability by allowing advanced specification of the upstream server, e.g. `https2http://example.com`
- Add UpstreamServerResolver to provide an unified interface for all non-default proxying modes.