Add `boot2docker shellinit`

Closes #2 

Add some minimal, and somewhat bad, test coverage to `boot2docker` and then add `boot2docker shellinit`, which can be evaluated to set the `DOCKER_HOST` environment variable.
This should be a real IP address, probably in `192.0.2.0/24`. (see http://tools.ietf.org/html/rfc5737)