added sitemap generation

[maplesteve]

Hi Marco!

I added the generation of a sitemap. It's written, when there's no sitemap file in htdocs or when a post or page was written. This relies on the files in the cache dir - hope I interpreted these files correctly.

Best regards, Stephan.

P.S.: Sorry for the three commits instead of one, but I struggled a bit to bundle my changes into your master base; my code base deviated over time a lot from the base version...