First pass at WP-native THATCamp Registry.

Features:
- Only logged-in users can access the page. Non-logged-in users are redirected
  back to the home page.
- Basic form validation: checks to make sure that required fields are filled
  in, and checks to make sure that the selected URL is valid and available
- Creates a new site and group automatically upon registration.
- Adds a new area on the THATCamp Setup screen where super admins can see
  additional registration information not otherwise displayed anywhere
  ("additional notes", twitter account, etc)

See #62. See #61.