title: AutoModelSelect2Field not working using django-select2
body: <p>I am using select2 for my dropdowns. I have dropdown that has some 18000 entries , so I want to use HeavySelect2Widget for it. I getting an error 
             <br/><b> Reverse of 'django_select2_central_json' with arguments () and keyword arguments '{}' not found</b><br/>
  Any idea what's the problem.The class and field name used is given below</p>
  
  <hr>
  
  <pre><code>class EmployeeChoices(AutoModelSelect2Field):
     fname = FirstName()
     queryset = [(1,'value 1')]#fname.getAllNames()
     search_fields = ['name__icontains',]
  </code></pre>
  
  <hr>
  
  <pre><code>field_name = EmployeeChoices(
      required=False,
      widget=AutoHeavySelect2Widget(
          select2_options={
              'placeholder': u"Select a choice"
          }
      ),
      label=''
  )
  </code></pre>