Make cookbook url validation stricter.

Use validate_url for Cookbook URL validations to ensure that no nonsense is
present.
Implement most downloaded/followed filters

Implement most downloaded/followed filters from cookbooks index. Also
add top three most downloaded/followed to cookbooks directory.
Merge pull request #245 from opscode/sort_by_downloaded_followed

Implement most downloaded/followed filters
Merge pull request #243 from opscode/strict_url_validations

Make cookbook url validation stricter.
Add category class to the cookbook partial.

This colors the cookbook partial based on their category.
Merge pull request #250 from opscode/cookbook_category_color

Add category class to the cookbook partial.
Format cookbook listing sort by options.
More usable setup for the create account/sign in functionality.
Variable-ize the user dropdown colors.
Add active state to categroy nav.
Media query tweaks to category nav active state.
Formatting adjustments to the cookbook urls buttons and form.
Format platform icons on cookbook show.
Some visual cleanup of the cookbook version dropdown on cookbook show.
Provide margin above the cookbook listing on category and search result pages.
Add bgcolor to category nav active state.
Downcase and underscore category names in category listing in order to render correct css class.
Change underscore parameterize on the category nav.