Remove constraint that Cask tokens cannot start with digits

This PR works by adding an internal prefix to all Cask class names.  Since the class names are now completely hidden from the user, the prefix is never seen.

This trick is considered to be an ugly transitional hack on the way to representing individual Casks as instances instead of classes.

Three Casks are also altered, but the change is backward-compatible and does not need to be coordinated with a code release.