Transfer and Approval event repo and repo tests + db migrations for these events.

db/migrations/20180827120358_create_token_transfer_table.down.sql

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+DROP TABLE token_transfers;

db/migrations/20180827120358_create_token_transfer_table.up.sql

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+CREATE TABLE token_transfers (
+  id                    SERIAL,
+  vulcanize_log_id      INTEGER NOT NULL UNIQUE,
+  token_name            CHARACTER VARYING(66) NOT NULL,
+  token_address         CHARACTER VARYING(66) NOT NULL,
+  to_address            CHARACTER VARYING(66) NOT NULL,
+  from_address          CHARACTER VARYING(66) NOT NULL,
+  tokens                DECIMAL NOT NULL,
+  block                 INTEGER NOT NULL,
+  tx                    CHARACTER VARYING(66) NOT NULL,
+  CONSTRAINT log_index_fk FOREIGN KEY (vulcanize_log_id)
+  REFERENCES logs (id)
+  ON DELETE CASCADE
+)

db/migrations/20180827120410_create_token_approval_table.down.sql

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+DROP TABLE token_approvals;

db/migrations/20180827120410_create_token_approval_table.up.sql

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+CREATE TABLE token_approvals (
+  id                    SERIAL,
+  vulcanize_log_id      INTEGER NOT NULL UNIQUE,
+  token_name            CHARACTER VARYING(66) NOT NULL,
+  token_address         CHARACTER VARYING(66) NOT NULL,
+  owner                 CHARACTER VARYING(66) NOT NULL,
+  spender               CHARACTER VARYING(66) NOT NULL,
+  tokens                DECIMAL NOT NULL,
+  block                 INTEGER NOT NULL,
+  tx                    CHARACTER VARYING(66) NOT NULL,
+  CONSTRAINT log_index_fk FOREIGN KEY (vulcanize_log_id)
+  REFERENCES logs (id)
+  ON DELETE CASCADE
+)