Initial implementation of creating users via invitation
fixed up strange issue of reviews controller tests being broken but rspec not seeming to run them when it should
working on feature testing for invitations
Fixed integration tests for invitations by setting host to www.example.com; also delete duplicate invitations when saving the latest one
Implementation where new invitation creation deletes all prior invitations with that target email
implemented different scheme for retiring invitations: so that after user registers, all invitations with tokens that match the registered email are deleted
implemented can_follow? logic and made use of this in user code.  Also some refactoring of invitation with delete_invitations_by_email implemented.
removed use of visit <path> in invitation integration tests and used explicit clicks to get to the proper pages
renamed the field used for user tokens (with original but abandoned intent of using Tokenable for user model); implemented Tokenable concern and used it in invitations;
example of Capybara sessions issue placed into tests
Settings changes for use of Mailgun; and minor formatting fixes for cleaner code review
Final tweaks based on Albert's code review.
Merge pull request #9 from yev2000/invite_users

Implementation of inviting users, along with use of Tokenable.  Made minor changes as per Albert's code review.