Add `boot2docker shellsetup`

Closes https://github.com/boot2docker/boot2docker-cli/issues/179

In this branch:

* Add the `shellsetup` command which outputs the correct environment variable export in order to use `boot2docker`. This can be used with `$ $(boot2docker shellsetup`.
* Some refactoring for unit testing.
* `virtualbox.Machine` dummy.

Upon acceptance there needs to be a documentation update, or at least an issue raised for one.