Fix for #2892.
1. Updatet the API documentation and parameter name from "object" to "sender" to reflect it's true purpose.
2. Fixed addObserver and observerExisted because it didn't allowed to add observers to notifications that only differs by sender
Fix for #2892.
3. Fixed registerScriptObserver method.
Merge pull request #3285 from gkosciolek/fix_for_CCNotificationCenter

closed 2611: Fixing the bug that observers with the same target and selector but different sender are the same observer in NotificationCenter.