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

pkg/transformers/pit_file/ilk/model.go

@@ -0,0 +1,9 @@
+package ilk
+
+type PitFileIlkModel struct {
+	Ilk              string
+	What             string
+	Data             string
+	TransactionIndex uint   `db:"tx_idx"`
+	Raw              []byte `db:"raw_log"`
+}