Include XML in sitemap.

Filter out known unwanted files (such as the sitemap itself).

Resolves issue #804.
Tests XML for root element rss instead of blacklisting paths
Merge branch 'master' of https://github.com/getnikola/nikola into include-rss-in-sitemap

Conflicts:
	CHANGES.txt
	nikola/plugins/task/sitemap/__init__.py
Whitespace before colon
Merge pull request #840 from Aeyoun/include-rss-in-sitemap

Include RSS in sitemap