Add support for docker-machine

[vidkun]

Docker Toolbox appears to be the new officially recommended method for installing Docker on OS X now. Additionally, boot2docker seems to be deprecated in favor of using the included docker-machine. In order to maintain functionality with these newer environment setups, docker-machine needs to be added.

Since adding this functionality means this project is no longer boot2docker specific, it would be more appropriate to rename the project to simply "docker-powify". I would recommend also renaming the repo to match. That would require updating your local clones with the new repo url: git remote set-url origin new_url

• Adds support for docker-machine which is included in the new Docker Toolbox.
• Maintains backward compatibility with boot2docker.
• Renames utilities to drop the "boot2" prefix.

This pull request closes #2