Add unix socket

While it may seem strange to have HTTP requests over a Unix socket, the possibility of it can't really be ruled out.

Projects like https://github.com/dotcloud/docker use a Unix socket instead of a port as default now.

This would open up the possibility of creating a Excon connection with the url of "unix:///var/run/docker.sock".  I've simply used the same branching originally used for regular or SSL sockets and added Unix socket.