Sitemap generator

This adds a sitemap generator to wagtail for automatically generating XML sitemaps for your site.

It currently only sets the 'location' and 'last_mod' attributes.

You can enable it by adding ```wagtail.contrib.wagtailsitemaps``` to ```INSTALLED_APPS``` and adding the following to your urls.py:

```
from wagtail.contrib.wagtailsitemaps.views import sitemap

urlpatterns = patterns('',
    ...

    url(r'^sitemap\.xml$', sitemap),
)
```