searching for subpages and 404 [2.5.1] (issue #107)

This version introduces multiple enhancements:

fixed problem when checking for 404 when net level is deeper

Details of additions/deletions below:
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Modified:   /BLUE_FRAMEWORK/packages/CORE/tree_class.php
            -- Updated --
                _tree method to don\'t unset $this->_mainPage when page wasn\'t found but set it to NULL
                some code refactor