Set associations between travelers, bookings, and schedules
Add schedules_controller with basic index
Add reserved integer to schedules
Schedules controller shows index and show
Add serializer for schedule
Add bookings_controller with create action
use a before action to do a post for testing
Replace booking with schedule_booker

The rationale is that booking shouldn't be responsible for updating the
schedule.reserved attribute, so I created a new object to handle that.
Its utility is debatable, because an alternative is to use an after_save
callback.