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
Fix for reassigning task to creator when owner is removed from Event
Fixes flow with bulk update operation including unchanged task owner
Merge pull request #75 from techiev2/events

Fix for reassigning task to creator when owner is removed from Event
Merge pull request #76 from techiev2/tasks

Fixes flow with bulk update operation including unchanged task owner
Merge pull request #77 from techiev2/invitations

Adds event invitation handles with accept/decline flow