Adding Configurable Display Depth

On some sites with deep category listings, you may not want to display the entire tree of a vocabulary. This patch adds a configurable option for the display depth on the sitemap.

I also quickly renamed what was called 'Category depth' but that was a little misleading anyway. Really that option was to configure the depth of the linked category page, not the display depth of the actual sitemap.

Please review and let me know if you have further ideas.