Restructure api and add event-status model

This is a baseline restructure of the Agency api to address a number of endpoint changes and incorporate feedback received in previous issue and PRs (#161, #116 and #63).

Major changes:
* `provider_id` is passed as part of an authorization bearer token.
* Looking up and registering vehicles use standard `GET` and `POST` operations on `/vehicles` endpoint.
* The `/vehicles/{vehicle_id}/event` allows Providers to `POST` events to affect vehicle status.  The vehicle status is modeled around a state flow that is modeled in Vehicle Events table.
* Deregister vehicle, trip start and end are now events.
* Events for `trip_enter` and `trip_leave` were added to address trips that do not start or end in the Agency jurisdiction.