Fix uncle getter

Uncle blocks don't necessarily have BlockDetails, so can't get total
difficulty directly.
Closes #1057

ethcore/src/client/ids.rs

@@ -53,6 +53,7 @@ pub struct TraceId {
 }
 
 /// Uniquely identifies Uncle.
+#[derive(Debug)]
 pub struct UncleId (
 	/// Block id.
 	pub BlockId,