title: Django-tables2 does not sort
body: <p>I display a database table using django-tables2. All appears OK, but clicking on the column headers does not sort by that column. The headers are clickable, but  in their html has no url text in it, e.g.:</p>
  
  <pre><code>&lt;div class="table-container"&gt;
  &lt;table class="paleblue"&gt;&lt;thead&gt;&lt;tr&gt;&lt;th class="id orderable sortable"&gt;&lt;a href=""&gt;ID&lt;/a&gt;
  &lt;/th&gt;&lt;th class="orderable sortable system"&gt;&lt;a href=""&gt;System&lt;/a&gt;&lt;/th&gt; ...
  </code></pre>
  
  <p>I checked the django-tables2 template source and it sais this:</p>
  
  <pre><code>... &lt;a href="{% querystring table.prefixed_order_by_field=column.order_by_alias.next %}"&gt; ...
  </code></pre>
  
  <p>Which I do not understand.</p>
  
  <p>I can only make sorting work by setting order_by in view.py:</p>
  
  <pre><code>class ResultsTable(tables.Table):
      class Meta:
          model = Performance
          attrs = {'class': 'paleblue'}
          order_by_field = True
  
  def result(request, system):
      results = Performance.objects.filter(system__name=system)
      table = ResultsTable(results, order_by=('&lt;any column name from Performance&gt;',))
      RequestConfig(request).configure(table)
      return render_to_response('result.html', {'table': table})
  </code></pre>
  
  <p>but that obviously only works for one column and I want to click on columns to choose which one to sort by.</p>
  
  <p>It is my understanding from the <a href="http://django-tables2.readthedocs.org/en/latest/" rel="nofollow">docs</a> that sorting by column should work 'out of the box', is this correct, or am I doing something else wrong?</p>