title: Django tables 2 exclude doesn't work
body: <p>I'm using Django 1.8.7 with djangotables2 1.3.0.
  I'm trying to build a table using ModelForm (example below). When I try to hide a column using exclude, it still shows all columns. Exclude doesn't seem to have any effect here.</p>
  
  <p>A part of my models.py:</p>
  
  <pre><code>class Event(models.Model):
      Name = models.CharField (max_length = 45, blank = False, null = False)
      Date = models.DateTimeField (auto_now_add=True)
      Text = models.CharField (max_length = 64, blank = False, null = False)
      class Meta:
          ordering = ["-Date", "Name"]
          verbose_name = "Event"
          verbose_name_plural = "Events"
          def __str__(self):
              return self.Name
  
  
  class Event_Form(ModelForm):
      class Meta:
          model=Event
          exclude = ('Date',)
  </code></pre>
  
  <p>A part of my views.py:</p>
  
  <pre><code>def dbtest_event(request,):
      return render(request, 'dbtest_event.html', {"Title":"Events",
      'dbtest_event':Event.objects.all()})
  </code></pre>
  
  <p>And the html-file that uses the table:</p>
  
  <pre><code>{% extends "dbtest.html" %}
  {% load render_table from django_tables2 %}
  {% load static %}
  {% block content %}
      {% render_table dbtest_event %}
  {% endblock %}
  </code></pre>
  
  <p>Thanks in advance for any help or ideas!</p>