Rework and restyle the cookbook directory.

This refreshes the content of the cookbook directory by adding a three column
view of cookbook listings and a section of helpful content.

The cookbook directory introduces a new style for the Supermarket stats which is
also reflected on the welcome page.
Merge pull request #354 from opscode/cookbook_directory_update

Rework and restyle the cookbooks directory.
fix a poor default in rubocop
add semverse gem
order versions semantically
validate that the version is semantically correct
enable our extensions before a test run, if necessary
correct a variety of bad assumptions in specs based on version ordering