Cookbook adoption

:convenience_store: 

This adds a way for a cookbook owner or Supermarket admin to mark a cookbook as up for adoption.  People interested in adopting the cookbook can click an "Adopt me!" button that appears on the cookbook details page when a cookbook is marked as up for adoption.  Clicking that button will send an email to the cookbook owner, alerting them that someone's interested, providing the email address of the interested party, and prompting them to get in touch.

Closes https://github.com/opscode/supermarket/issues/829