'ancestors', 'descendants' method fix, added 'children' method
'ancestors', 'descendants' method fix, added 'children' method

Now 'ancestors' method works properly and returns all of the entity's
ancestors reversely ordered by their level. 'Descendants' method now
returns all of the entity's descendants, not only its immediate
children. And new method 'children' returns exactly latter.

ramirezd42/ClosureTable,rafis/ClosureTable,EspadaV8/ClosureTable,franzose/ClosureTable