Adds base Invitation data model
Adds InvitationController and updates url map
Adds Invitation.json and Invitation.get helper method
Adds POST handler method for InvitationController
Updates Invitation.json to instance method and updates Invitation.modify
Merge pull request #77 from techiev2/invitations

Adds event invitation handles with accept/decline flow
Check to show non-signed up members as "Invite to Haywire" from User.valid_users
Merge pull request #78 from techiev2/users

Check to show non-signed up members as "Invite to Haywire" from User.val...