Add support for unix domain sockets

adapted from some code from [docker-py](https://github.com/docker/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.

#### Screenshot

![screen shot 2014-11-24 at 6 35 24 pm](https://cloud.githubusercontent.com/assets/305268/5177115/dba014c6-7408-11e4-8376-78a57a39909b.png)

Fixes: GH-209 (https://github.com/jakubroztocil/httpie/issues/209)

Cc: @shin-, @jakubroztocil, @monsanto, @np, @nuxlli, @matrixise, @remmelt
See https://github.com/jakubroztocil/httpie/pull/276#discussion_r20866010