fix function names and dotted lookups for use in PrimaryKeyRelatedField

I've been using this, working fine:
    
    class ExportSerializer(serializers.ModelSerializer):
        #...
        layers = serializers.RelatedField(many=True, source='get_layers', queryset=ExportLayer.objects.all())

This works fine, but then I tried to use `serializers.PrimaryKeyRelatedField` instead and everything broke:


    Traceback:
    File "/home/cdestigter/checkout/django/django/core/handlers/base.py" in get_response
      115.                         response = callback(request, *callback_args, **callback_kwargs)
    File "/home/cdestigter/checkout/django/django/views/generic/base.py" in view
      68.             return self.dispatch(request, *args, **kwargs)
    File "/home/cdestigter/checkout/django/django/views/decorators/csrf.py" in wrapped_view
      77.         return view_func(*args, **kwargs)
    File "/home/cdestigter/checkout/django-rest-framework/rest_framework/views.py" in dispatch
      404.             response = self.handle_exception(exc)
    File "/home/cdestigter/checkout/django-rest-framework/rest_framework/views.py" in dispatch
      401.             response = handler(request, *args, **kwargs)
    File "/usr/local/lib/python2.6/dist-packages/poppy/exports/views.py" in get
      19.         return Response(serializer.data)
    File "/home/cdestigter/checkout/django-rest-framework/rest_framework/serializers.py" in data
      387.                 self._data = [self.to_native(item) for item in obj]
    File "/home/cdestigter/checkout/django-rest-framework/rest_framework/serializers.py" in to_native
      279.             value = field.field_to_native(obj, field_name)
    File "/home/cdestigter/checkout/django-rest-framework/rest_framework/relations.py" in field_to_native
      226.             return [self.to_native(item.pk) for item in queryset.all()]

    Exception Type: AttributeError at /api/v2/exports/
    Exception Value: 'function' object has no attribute 'all'


This PR fixes the error.