Extend run config to let users specify an IP

Added an option '--ip' to 'docker run' to be able to statically specify the IP for a container. This change helps get around the problem of IP changing across container restarts.
@thockin.
Another test that fails on my system. [`hostConfig`](https://github.com/vishh/docker/blob/a27bebc493527e29d8291d1c364e423ca316098d/runconfig/hostconfig.go#L22) has no member called `IP`.