Sitemap improvements

I finally did it! :) But I do not like the way the plugin written. There is a great potential to code improvements here. Maybe someday when I'll have a lot of free time I'll do it.

Sitemap plugin improvements:

* use Jinja2 to rednering sitemap;
* detect last modification from the fileystem;
* add `None` to the valid values;
* add `generate` key to the sitemap config;
* pep8.