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.
On all functions, typical Go style is to put a comment immediately before the `func` line starting with the name of the function, and not embed a multi-line comment within the function itself.  For example:

```go
// DockerHostExportComand returns the 'export' command to set the DOCKER_HOST env var.
func DockerHost...
```