Add SMTP configuration to example .env
Add nokogiri as a dependency

Premailer requires nokogiri or hpricot as a dependency. It includes
neither as it lets the developer to decide which they prefer so we must
include one, I choose nokogiri.
Don't require raven

We should only require raven if a sentry url is provided which we're
doing in config/raven.rb. This removes it from being required by
bundler.
Merge pull request #254 from opscode/fix_premailer

Fix premailer
More usable setup for the create account/sign in functionality.
Revise Cookbooks UI

- Format cookbook listing sort by options.
- Variable-ize the user dropdown colors.
- Add active state to category 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.
Extract link_to_sorted_cookbooks helper