Add replay protection (#4808)

* add eip155

* make network_id default

ethcore/src/engines/mod.rs

@@ -166,7 +166,9 @@ pub trait Engine : Sync + Send {
 	}
 
 	/// The network ID that transactions should be signed with.
-	fn signing_network_id(&self, _env_info: &EnvInfo) -> Option<u64> { None }
+	fn signing_network_id(&self, _env_info: &EnvInfo) -> Option<u64> {
+		Some(self.params().chain_id)
+	}
 
 	/// Verify the seal of a block. This is an auxilliary method that actually just calls other `verify_` methods
 	/// to get the job done. By default it must pass `verify_basic` and `verify_block_unordered`. If more or fewer