Improvements to metadata

No actual features or bug fixes in this pull request, but I wanted to offer some small improvements :smile:

Check out the commit messages for full explanations, but here's a summary:

* Re-vamp the README: add introduction, syntax highlighting, improve examples, etc.
* Add comments to the code (hopefully will make it easier for others to contribute)
* Add a description to the Vagrant plugin definition
* Raise a friendly error when used outside of Vagrant (incorrect installation)
* Expand the Gem description (made it longer), and made the summary shorter
* Add `rake` and `bundler` as development dependencies
* Remove trailing whitespace

I actually started my own Babushka provisioner for Vagrant, before checking if anybody had already done it. I only found out when I tried to push the finished gem to Rubygems! :disappointed: So this means I have some further improvements to contribute soon!