refactored the class and classbase classes, and introduced a classimpl class in the src directory, this is a first step to remove all zend-specific code from the header files, so that we can later have a hhvm backend for the php-cpp library
added Class::implements() method to allow classes that implement interfaces (request from issue #52)
it now is possible to register a class as base class (although not yet working) (request from issue #52)
base classes can be added too, with the class::extends() method (request from issue #52)