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);

db/schema.sql

@@ -2,8 +2,8 @@
 -- PostgreSQL database dump
 --
 
--- Dumped from database version 10.0
--- Dumped by pg_dump version 10.0
+-- Dumped from database version 10.1
+-- Dumped by pg_dump version 10.1
 
 SET statement_timeout = 0;
 SET lock_timeout = 0;
@@ -177,6 +177,14 @@ ALTER TABLE ONLY blocks
     ADD CONSTRAINT blocks_pkey PRIMARY KEY (id);
 
 
+--
+-- Name: watched_contracts contract_hash_uc; Type: CONSTRAINT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY watched_contracts
+    ADD CONSTRAINT contract_hash_uc UNIQUE (contract_hash);
+
+
 --
 -- Name: schema_migrations schema_migrations_pkey; Type: CONSTRAINT; Schema: public; Owner: -
 --