allow tcp:// and http:// URLs in $DOCKER_HOST

Allowing `DOCKER_HOST` to contain URLs like `tcp://192.168.59.103:2375` and `http://192.168.59.103:2375`.

In particular, boot2docker (for Mac OS X) sets the `DOCKER_HOST` environment variable using the `tcp:` protocol, so decking should now work out of the box for Mac users.

Nice project!