Switch to Pod::Simple::XHTML

I strongly recommend Pod::Simple::XHTML over Pod::Simple::HTML. The HTML it generates is much cleaner, and you don't have to strip out the header and footer like you did with the hack that was there before. The one downside is that Pod::Simple 3.11 is required for it to be valid XHML (a bunch of bugs were fixed in that version). But overall it's just much cleaner to use.