Fixed IDisposable implementation and usage.

- Implement IDisposable interface correctly, with sealed classes where possible for simplicity.
- Add using statement around undisposed local variables.

OpenRA.Game/Download.cs

@@ -70,6 +70,7 @@ namespace OpenRA
 		{
 			Game.OnQuit -= Cancel;
 			wc.CancelAsync();
+			wc.Dispose();
 			cancelled = true;
 		}
 	}