Moving callback firing code into its own method.

[jamesmoss]

I've just refactored the end method, pulling out the code that creates a SimpleXMLElement and calls call_user_func_array into it's own method named fireCallbacks.

My use case for this is that I want to use xmlStreamReader to count the number of times a path is matched in large XML docs. Speed is important so I don't want the overhead of creating a SimpleXMLElement object and calling call_user_func_array everytime a path is matched. In my own class I'm planning to extend Parser and override fireCallbacks to increment a counter var instead.

[hobnob]

@jamesmoss Cool, looks nice. Thanks! :-)