Change page url functions to use reverse

Fixes issue #69
Merge branch 'page_url' of https://github.com/nathan3d/wagtail into nathan3d-page_url
use 'wagtail_serve' as the url name rather than 'wagtail'
Add tests for urlconfs that have wagtail_serve at a non-root path
changelog entry for #531
Merge branch 'nathan3d-page_url'