RPC & Client (#21)

* Initial version of http server.

* Refactor the structure.

* add unassigned ports info.

* Rename servers to match conventions.

* Add client crate and some proper RPC implementations.

* Style & docs.

substrate/state_machine/src/backend.rs

@@ -16,7 +16,7 @@
 
 //! State machine backends. These manage the code and storage of contracts.
 
-use std::fmt;
+use std::{error, fmt};
 
 use primitives::Address;
 use primitives::hash::H256;
@@ -60,6 +60,10 @@ impl fmt::Display for Void {
 	}
 }
 
+impl error::Error for Void {
+	fn description(&self) -> &str { "unreachable error" }
+}
+
 /// In-memory backend. Fully recomputes tries on each commit but useful for
 /// tests.
 #[derive(Default)]