Excel5 writer with optional constructor parameter

Hi,

_I ran into an issue where I need to create a writer, then pass it without knowing the PHPExcel object._

I think it would be better to have both Excel5 and Excel2007 writer working the same way.
So, I add a setter and getter, and change every access to "__phpExcel" by the getter. This one throw an exception if no PHPExcel object is set.