(Yaf >=1.0.0) Retrieve an item from registry

(Yaf >=1.0.0) The __clone purpose private

(Yaf >=1.0.0) Yaf_Registry implements singleton

(Yaf >=1.0.0) Remove an item from registry

(Yaf >=1.0.0) Add an item into registry public

(Yaf >=1.0.0) Check whether an item exists