Add support for unix domain sockets

adapted from some code from docker-py:

* https://github.com/docker/docker-py/blob/master/docker/unixconn/unixconn.py

Also submitted this UnixAdapter code to `requests` as
https://github.com/kennethreitz/requests/pull/2355, but this PR was [declined](https://github.com/kennethreitz/requests/pull/2355#issuecomment-64299663), as the `requests` maintainers do not want to maintain this feature.

Fixes: GH-209 (https://github.com/jakubroztocil/httpie/issues/209)