Add support for proxies running on a Unix domain socket
Add `proxy_socket` to list of valid connection keys
Add parameter / environment / connection tests for Unix socket proxies
Switch unix socket tests to pending under JRuby
Remove proxy socket style option; go with `unix:///tmp/myproxy.socket` format
Merge pull request #427 from brandur/brandur-unix-proxies

Add support for proxies running on a Unix domain socket