KeyStore implementation + key derivation (#97)

* improve ed25519 bindings

* probably broken child derivation

* basic keystore

* keystore integration in CLI

* constant-time mac comparison

* fix spaces

polkadot/cli/src/error.rs

@@ -26,4 +26,11 @@ error_chain! {
 	links {
 		Client(client::error::Error, client::error::ErrorKind) #[doc="Client error"];
     }
+	errors {
+		/// Key store errors
+		Keystore(e: ::keystore::Error) {
+			description("Keystore error"),
+			display("Keystore error: {:?}", e),
+		}
+	}
 }