Add unix socket and multiple -H

-H now can be (if no scheme, tcp is assumed):
* `tcp://host` -> tcp connection on host:4243
* `tcp://host:port` -> tcp connection on host:port
* `tcp://:port` -> tcp connection on 127.0.0.1:port
* `unix://path/to/socket` -> unix socket located at path/to/socket

You can also  listen to multiple host/socket:

`docker -H tcp://127.0.0.1:4243 -H tcp://0.0.0.0:4444 -H unix:///var/run/docker.sock -d`

/cc @shykes @creack @benoitc