State pruning (#216)

* Started work on state db

* Updated for a new hash type

* Pruning and tests

* Generalize on the block hash/key type

* Integrate with the client backend

* Merge w master

* CLI options

* Updated for light client refactoring

* Used IntoIterator

* Fixed invalid input hadling

polkadot/cli/src/error.rs

@@ -28,5 +28,10 @@ error_chain! {
 		Client(client::error::Error, client::error::ErrorKind) #[doc="Client error"];
     }
 	errors {
+		/// Input error.
+		Input(m: String) {
+			description("Invalid input"),
+			display("{}", m),
+		}
 	}
 }