Add description to plugin class
Improve clarity and detail in README

- Add introduction section with links to Vagrant/Babushka
- Convert URLs to links
- Use syntax highlighting in code segments
- Improve clarity of usage examples, add example for dep options
- Space sections out (improves clarity when editing/reading in
  plaintext)
Improve message when trying to use outside Vagrant

If the plugin is installed as a Gem outside Vagrant, some files can't
be loaded and will raise an error anyway. We might as well improve the
message to make it clear this isn't supported.
Shorten Gem summary; lengthen description
Improve internal documentation

- Copy comments from inherited methods, as a reminder of their purpose
- Add comments to custom methods, including descriptions for parameters
Add rake and bundler as development dependencies

Although most people will already have these commands on their system,
technically they are required for development, and should be specified
in the Gem specification.