Updated page_url tag to follow design decision

The agreed upon logic is that using the tag without the 'as' breakpoint, uses the backwards compatible logic that will raise a Page.DoesNotExist exception when the page cannot be found AND when settings.DEBUG=True.

Using the tag with the 'as' breakpoint will always suppress the Page.DoesNotExist exception regardless of the settings.DEBUG value.

Also, to do this cleanly, Ojii has updated django-classy-tags to expose an overridable get_value_for_context() method on AsTag. This bumps up the version required to 0.5 for django-cms.  setup.py and test-requirements/* have been updated to reflect this.

Also, there are now rather thorough tests "protecting" this design decision from future changes.