Handle non-field errors from Django

Django uses django.core.exceptions.NON_FIELD_ERRORS or "__all__" for
identifying non-field errors.  This is returned when a ValidationError
is raised in the model's clean method, or if a unique_together
constraint fails.
Add support for DRF-style pagination

When the list is paginated (paginate_by = N), the pagination data will
be added to the meta section:

{
    "data": [{
        "name": "some paginated data",
    }],
    "meta": {
        "pagination": {
            "data": {
                "count": 1,
                "next": "http://example.com/data?page=2",
                "previous": null,
            }
        }
    }
}

This is keyed so that future code can include other, not pagination
data, and so it would be possible to paginate related links using keys
similar to the "links" section.
Merge pull request #15 from jwhitlock/fix_django_non_field_errors

Handle non-field errors from Django
Merge pull request #16 from jwhitlock/add_pagination

Add support for DRF-style pagination