Success to use singleton AuthenticationHelper.
Add plan-show view.
Add attend action.
Add PlanSchedule.
Change model structures.
Add resources.
Add user own resources.
Add backend OPTIONS access.
Modify resource __parent__ structure.
Change library files' structure.
Modify POST attendants and comments in Plan.
Modify traversal structure to pure traversal.
Modify API routing.
Implement API.
Delete commented out codes.
Modify backend's exceptions, ndb properties and models.
Return User model values in PlanAttendant resource.
Refactor resources.
Add me/plans API views.
Remove obsoleted files.