Collect consensus/null engines into a single module (#1754)

* collect consesnsus engine code into module

* move Engine to mod

* fix json test

ethcore/src/client/client.rs

@@ -40,7 +40,7 @@ use header::BlockNumber;
 use state::State;
 use spec::Spec;
 use basic_types::Seal;
-use engine::Engine;
+use engines::Engine;
 use views::HeaderView;
 use service::ClientIoMessage;
 use env_info::LastHashes;