* 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.
NSJSONSerialization parses long values as doubles and returns NSNumber that are backed by doubles to the users. For NSNumbers that can be stored as longs, we can overwrite this behavior by re-running the initialization step.