move dataframe creation code to view?

[sheppard]

DRP currently requires the use of a custom serializer.data method that returns a Pandas dataframe. It might be more flexible to allow whatever serializer people want to use and do the DataFrame creation & manipulation entirely on the view. This would likely mean overridding view.list() and adding a call to a new self.create_dataframe(serializer.data)

[sheppard]

DataFrame creation is now handled by a separate pandas_serializer_class, which corresponds to the new list_serializer_class in DRF 3 (#10). For backwards compatibility with DRF 2.4, this class is mixed in to any user-defined serializer class on the view.

This change makes it easier to define custom non-DRP serializers and still benefit from DRP.