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

db/migrations/1535667935_create_pit_file_table.down.sql

@@ -1 +1,2 @@
-DROP TABLE maker.pit_file;
+DROP TABLE maker.pit_file_ilk;
+DROP TABLE maker.pit_file_stability_fee;

db/migrations/1535667935_create_pit_file_table.up.sql

@@ -1,10 +1,20 @@
-CREATE TABLE maker.pit_file (
+CREATE TABLE maker.pit_file_ilk (
   id            SERIAL PRIMARY KEY,
   header_id     INTEGER NOT NULL REFERENCES headers (id) ON DELETE CASCADE,
   ilk           TEXT,
   what          TEXT,
-  risk          NUMERIC,
+  data          NUMERIC,
 	tx_idx        INTEGER NOT NUll,
   raw_log       JSONB,
   UNIQUE (header_id, tx_idx)
-);
+);
+
+CREATE TABLE maker.pit_file_stability_fee (
+  id SERIAL PRIMARY KEY,
+  header_id INTEGER NOT NULL REFERENCES headers (id) ON DELETE CASCADE,
+  what      TEXT,
+  data      TEXT,
+  tx_idx    INTEGER NOT NULL,
+  raw_log   JSONB,
+  UNIQUE (header_id, tx_idx)
+)