For #194, convert the POTD management command to a celery task. Refactored to put the logic for the command into a function, and the command simply calls this function. The task can also just call this function. Added some basic tests for the new function.

{% load core_tags %}
<p>
<span class="shoutbox-user">{{ shout.user.username }}:</span>
<span class="shoutbox-shout">{{ shout.html|safe }}</span><br />
<span class="shoutbox-date">{{ shout.shout_date|elapsed }}</span>
</p>