Add Pit file stability fee event

- file event on the pit contract is overloaded and each implementation
  logs a note
- existing Pit file transformer now specified as dealing with ilks

test_config/test_config.go

@@ -77,7 +77,8 @@ func CleanTestDB(db *postgres.DB) {
 	db.MustExec("DELETE FROM headers")
 	db.MustExec("DELETE FROM log_filters")
 	db.MustExec("DELETE FROM logs")
-	db.MustExec("DELETE FROM maker.pit_file")
+	db.MustExec("DELETE FROM maker.pit_file_ilk")
+	db.MustExec("DELETE FROM maker.pit_file_stability_fee")
 	db.MustExec("DELETE FROM receipts")
 	db.MustExec("DELETE FROM transactions")
 	db.MustExec("DELETE FROM watched_contracts")