fix UrlNodeAdminForm, allow override_url to be missing
BIC: added html_title to HtmlPage/SeoPageMixin model.

- You'll have to add a south migration for your custom page types
  that inherited from HtmlPage/FluentPageBase/AbstractFluentPage.
setup: expliclty rely on 1.0a1 version for django-parler

Parts of the git version are used, and will need to be 1.0
before django-fluent-pages 0.9 can be released.