Clear cache properly when saving a CMS page

Extension by default overrides the cms page model to change the logic,
rather than add new logic. As a result, it doesn't call the original
model logic. Unfortunately, this means it doesn't call
Mage_Core_Model_Abstract::_afterSave(); which means that it doesn't
trigger the after save events, or the cache clears.

This commit bypasses the parent::_afterSave() event, and instead calls
Mage_Core_Model_Abstract::_afterSave() itself.