Add active() method on UserManager & documentation

Added the active() manager method that can be called across relatedmanagers. enables following syntax in templates:

    {% for moderator in forum.moderators.active %}
        {{ moderator|profilelink }}{% if not forloop.last %},{% endif %}
    {% endfor %}

instead of 

    {% for moderator in forum.moderators.all %}
        {% if moderator.active %}
            {{ moderator|profilelink }}{% if not forloop.last %},{% endif %}
        {% endif %}
    {% endfor %}