Add callback support for "AjaxFormHelper::transitEvent" method

Class AjaxFormHelper is used to transform even result into a valid JSON response, that later can be processed by FormManager class (in JavaScript), that originally was used to issue that even call, that invoked AjaxFormHelper in first place.

Main method in that class is transitEvent and after using it for a while we developed a few additions to it, that might come in handy:

• closure support - ability to specify PHP 5.3+ closure, that would be called after event was executed, but before output is sent out in JSON format
• return event status - return event invocation status to code, where transitEvent method was used