Merge pull request #504 from ethcore/rpc_poll_ids

support for rpc polling

sync/src/tests/helpers.rs

@@ -105,6 +105,10 @@ impl BlockChainClient for TestBlockChainClient {
 		Some(U256::zero())
 	}
 
+	fn block_hash(&self, id: BlockId) -> Option<H256> {
+		unimplemented!();
+	}
+
 	fn code(&self, _address: &Address) -> Option<Bytes> {
 		unimplemented!();
 	}