Add sitemap generating events from 1.9.0.1

In 1.9.0.1 two events were added to Sitemap/Model/Sitemap.php generateXml():

sitemap_categories_generating_before
sitemap_products_generating_before

They allow the respective collections to be modified before the xml is created.
This patch copies those events into Creare_CreareSeoCore_Model_Sitemap_Sitemap.
Merge pull request #8 from rbrown/sitemap_changes

Add sitemap generating events from 1.9.0.1