Add disqus_sso tag

So users who are logged into the Django site can post comments to Disqus
without having to log in to disqus separately
change gitmodule URL to public URL
Simplify templatetag registration, allow setting disqus_category_id.

Automatically merge multiple arguments passed to disqus_url templatetag, as
this allows you to write:

    {% set_disqus_identifier request.get_host "/" unique_object_id %}
    {% set_disqus_url "http://" request.get_host request.path %}

in your templates and have them concatenated to valid identifiers and URLs
automatically, without writing any code in your view.
Merge branch 'master' of git://github.com/aptivate/django-disqus