Allow deliberate None values
Allow deliberate None values

Closes #132

pivotal-energy-solutions/django-datatable-view,jangeador/django-datatable-view,doganmeh/django-datatable-view,pivotal-energy-solutions/django-datatable-view,doganmeh/django-datatable-view,jangeador/django-datatable-view,doganmeh/django-datatable-view,jangeador/django-datatable-view,pivotal-energy-solutions/django-datatable-view