Rework the sitemap module a bit.
Rework the sitemap module a bit.

- Consider all pages that are not a redirect instead of just the pages in_navigation.
- Improve the way page priority is calculated: in addition to the level decreasing the priority, also boost the prio if a page is in_navigation and thus "more important".
- Compute constant stuff once and not for each page (max_level)
- Round the priority to 2 decimals
- Make the modification_date access not fail if no changedate extension is registered. Should fix issue #114.
mjl/feincms,mjl/feincms,matthiask/django-content-editor,hgrimelid/feincms,pjdelport/feincms,matthiask/feincms2-content,joshuajonah/feincms,feincms/feincms,michaelkuty/feincms,michaelkuty/feincms,feincms/feincms,nickburlett/feincms,michaelkuty/feincms,pjdelport/feincms,joshuajonah/feincms,mjl/feincms,joshuajonah/feincms,feincms/feincms,hgrimelid/feincms,matthiask/django-content-editor,matthiask/django-content-editor,nickburlett/feincms,nickburlett/feincms,matthiask/feincms2-content,matthiask/django-content-editor,michaelkuty/feincms,nickburlett/feincms,joshuajonah/feincms,hgrimelid/feincms,matthiask/feincms2-content,pjdelport/feincms