Fix #250

• Fix #250 (closed) by adding the RPC module for Grandpa.

Note: proveFinality call will only work after the first authority set change.

node/src/rpc.rs

@@ -149,6 +149,7 @@ where
         )
         .into_rpc(),
     )?;
+
     module.merge(System::new(client.clone(), pool, deny_unsafe).into_rpc())?;
     module.merge(TransactionPayment::new(client).into_rpc())?;
     if let Some(command_sink) = command_sink_opt {