sitemap job refactoring to improve extensibility

I've had some issues when I wanted to override the sitemap job.
All I wanted to do is to add some custom entries without overriding the whole file.

Unfortunately most of the logic was in run(), a method I didn't want to override.

I created a new method "AddPages" which you can override. At this point you can easily add additional entries to the xml file, no need to override the whole run method or remove </urlset> first