Add support for unix domain sockets

To issue HTTP requests to a UNIX domain socket:

  * Use new "http+unix://" scheme in URL.
  * Use urlencoded socket path as the hostname part of the URL.
    e.g.: for a socket at /tmp/profilesvc.sock, you could do:

    http http+unix://%2Ftmp%2Fprofilesvc.sock/status/pid

This uses https://pypi.python.org/pypi/requests-unixsocket

Fixes: GH-209 (https://github.com/jakubroztocil/httpie/issues/209)