Add `dvm ip` command to output the address of the Vagrant machine

This adds a helper to print the configured (or default) IP address
of the Vagrant VM. Similar to `boot2docker ip`, except just reads the
configuration and doesn't try to connect to the machine itself.

Example use case would be to connect to published container ports,
like:

    dvm up
    docker run -d -p 80:80 my_web_server
    curl -i "http://$(dvm ip)/foo"
Use the new `docker_ip` function in `setup_env`

DRY
Merge pull request #46 from tmatilai/dvm_ip

Add `dvm ip` command to output the address of the Vagrant machine