Operator and OperatorServingStop data model
Operator's each have a OnestopID
Identifier's are now generic models, which can be (polymorphically) associated with Stop's or Provider's
model concerns

* organizing OnestopID methods and constants into a service and a model concern
* renaming model concerns to be consistent
* adding a named scope for searching by identifier
shorten name to `Stop.match_against_existing_or_create`
API endpoints for Operator's
StopController#index shouldn't try to preload Operator's into that mega SQL query
making StopsController tests a bit more compact
adding oj gem for faster JSON rendering & updating airborne JSON testing gem
adding an API endpoint for looking up any type of entity by its OnestopID

it can be a Stop or an Operator
Operator OnestopID's should have a geohash of 2 characters in length
Merge pull request #27 from transit-land/operators

add Operator and OperatorServingStop