Merge branch 'master' into auth-round

ethcore/src/engines/mod.rs

@@ -112,6 +112,9 @@ pub trait Engine : Sync + Send {
 	/// Verify a particular transaction is valid.
 	fn verify_transaction(&self, _t: &SignedTransaction, _header: &Header) -> Result<(), Error> { Ok(()) }
 
+	/// The network ID that transactions should be signed with.
+	fn signing_network_id(&self, _env_info: &EnvInfo) -> Option<u8> { None }
+
 	/// 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
 	/// methods are needed for an Engine, this may be overridden.