Merge pull request #10 from alim/Issue_4

Issue #9 Update admin layout in preparation for Rails Admin integration
Issue #11 - Testing validation updates to the User model
Issue #11 - Added parameter sanitizer for Devise controller signup action.
Issue #11 - Updated validations for User model to include last_name and phone
Issue #4 - Fixed issues with index view and dependent destruction of a group, when a user is deleted.