title: type object 'SignupForm' has no attribute 'validate'
body: <p>I'm extending de User predefined of the Django Auth. With this:</p>
  
  <pre><code>#models.py
  class Estudiante(models.Model):
     user    = models.OneToOneField(User)
     carrera = models.IntegerField('carrera', choices=CARRERA_CHOICES)
     carnet  = models.CharField(max_length=8)
     horas   = models.IntegerField('horas')
  
  
  class SignupForm(forms.Form):
    carrera = models.IntegerField('carrera', choices=CARRERA_CHOICES)
    carnet  = models.CharField(max_length=8)
  
  class Meta:
     model = Estudiante
     fields = ('carrera', 'carnet')
  
  def save(self, user):
      user.carrera = self.cleaned_data['carrera']
      user.carnet = self.cleaned_data['carnet']
      user.save()
  
  
  #admin.py
  admin.site.register(Estudiante, SignupForm)
  </code></pre>
  
  <p>And I'm receiving this error when I request the login page:</p>
  
  <pre>
  AttributeError at /
  type object 'SignupForm' has no attribute 'validate'
  Request Method: GET
  Request URL:    localhost
  Django Version: 1.6.2
  Exception Type: AttributeError
  Exception Value:    
  type object 'SignupForm' has no attribute 'validate'
  </pre>
  
  <p>I'm using AllAuth, Django 1.6</p>