Get ABI via etherscan API (#96)

- Added ABI request
- Add unique constraint on contract hash for watched contracts

db/migrations/1512595007_add_contract_hash_constraint_to_watched_contracts.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE watched_contracts
+  DROP CONSTRAINT contract_hash_uc;

db/migrations/1512595007_add_contract_hash_constraint_to_watched_contracts.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE watched_contracts
+  ADD CONSTRAINT contract_hash_uc UNIQUE (contract_hash);