Use get_fields() instead of deprecated get_all_field_names
Use get_fields() instead of deprecated get_all_field_names

Modified to user get_fields since get_all_field_names is deprecated in
Django 1.8.
This also fixes proxied models.

doganmeh/django-datatable-view,annaproch/django-datatable-view,pivotal-energy-solutions/django-datatable-view,greenbender/django-datatable-view,pivotal-energy-solutions/django-datatable-view,addgene/django-datatable-view,addgene/django-datatable-view,pivotal-energy-solutions/django-datatable-view,jangeador/django-datatable-view,greenbender/django-datatable-view,jangeador/django-datatable-view,annaproch/django-datatable-view,greenbender/django-datatable-view,jangeador/django-datatable-view,annaproch/django-datatable-view,doganmeh/django-datatable-view,addgene/django-datatable-view,doganmeh/django-datatable-view