Cache the sitemap

Use SilverStripe partial caching to boost performances on sites where
generating the sitemap is costly (such as on http://gtkdoc.entidi.com/).

These are the peformance results on the local aforementioned website:

ab -n100 -c 10 http://gtkdoc.entidi.com.local/

			Time		Request per seconds
uncached		8.965		11.15
cached			3.934		25.42