Fix for compatibility problem with PHP 5.3.2

Resolution for issue https://github.com/garygolden/markdown-oo-php/issues/20

The solution is to define a constructor for the `Filter` abstract class.  This addition has no effect on the code whatsoever, as it is a base abstract class, and the constructor is empty, so I assume the problem is in PHP.  The problem was reproduced on multiple Linux boxes running PHP 5.3.2, and seems to be fixed before PHP 5.3.5.