Add `boot2docker shellsetup`

Closes #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)`. The output looks like `export DOCKER_HOST=tcp://ip:port`
* 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.

**Note**: I am unhappy with the name of this command, suggestions are welcome.
None of your functions return `error`, yet all of them could potentially fail and/or return unusable results.
Returning an `int` as status code is not idiomatic Go.