Add Pep price feed

db/migrations/1532533671_create_peps_table.down.sql

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+DROP TABLE maker.peps;
+DROP SCHEMA maker;

db/migrations/1532533671_create_peps_table.up.sql

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+CREATE TABLE maker.peps (
+  id           SERIAL PRIMARY KEY,
+  block_number BIGINT  NOT NULL,
+  header_id    INTEGER NOT NULL,
+  usd_value    NUMERIC,
+  CONSTRAINT headers_fk FOREIGN KEY (header_id)
+  REFERENCES headers (id)
+  ON DELETE CASCADE
+);