Add collaborators

Add collaborators feature- allow premium users with private wikis the ability to collaborate with other registered users.
  * View all public wikis for non-logged in users.
  * Users can view and edit only private wikis that are shared with them.
  * Integration tests passing.
  * Created seeds file for simplifying development.
  * Enhanced the private wiki feature so users can only see their private wikis.
Do not write to stdout. Use Rails' logger if you want to log.