* Delete an app from Database when deleted in Core.
When a FIRApp is deleted in Core, it should also be deleted from
the instances array in Database.
See #160 for more details.
* Add tests for deleting databases.
* Add call to clean up deleted FIRDatabase instance.
* Move Auth notification constant into Core.
SDKs that want to listen for the internal Auth notifications needed
to copy the notification strings to their own SDK instead of relying
on Auth's definition in order to avoid a dependency on Auth. By moving
them to Core, SDKs can use the constants without taking on another
dependency.
* Remove Auth stub, add app instance to notification.
Paul Beusterien