Added invitations controller and feature.
Renamed Invitations to Invitees.
New user invitation working up to viewing shared itineraries.
Newly invited users can add stops to itineraries shared with them.

Any logged in user can add stops to any itinerary.  MUST FIX.
Added spec for Itinerary.find_for_user.

Users can only add stops to their own itineraries and itineraries they have been invited to.
Users can invite existing users to itineraries.
Refactor
Added UserInviter and spec.  All specs and features passing.
Merge branch 'invitation_redux'