Save Logs to DB (#100)

* Save logs to database

* Save multiple logs to db

* Add block number parameter to `FindLogs`

db/migrations/1513029953_add_logs_table.down.sql

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+DROP TABLE logs;

db/migrations/1513029953_add_logs_table.up.sql

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+CREATE TABLE logs (
+  id           SERIAL PRIMARY KEY,
+  block_number BIGINT,
+  address      VARCHAR(66),
+  tx_hash      VARCHAR(66),
+  index        BIGINT,
+  topic0       VARCHAR(66),
+  topic1       VARCHAR(66),
+  topic2       VARCHAR(66),
+  topic3       VARCHAR(66),
+  data         TEXT,
+  CONSTRAINT log_uc UNIQUE (block_number, index)
+);
+