- Copy static valueOf() method to all subclasses and make base
  class version throw an exception. This fixes issue #293 - PHP 5.3.3 has
  a bug with the "static" keyword and class resolution which surfaces when
  invoking this method reflectively.
# Actually a good idea anyway, since we can now implement tougher checks
# in subclasses, currently "new Integer(28.4)" works, which it should not!