WebAPIResource now allows subclasses to set paginated_cls to a

subclass of a WebAPIResponsePaginated. This will be used for handling

the pagination of the resource's queryset and for serializing the data.
WebAPIResponsePaginated has been fully restructured to make its behavior

customizable. Subclasses can override nearly every aspect of it,

including previous/next calculation, URL building, results fetching,

link building, etc. This makes it possible to do such things as paginate

by SHA1, or by page (instead of result item index), or fetch data from

something other than a database-backed queryset.

All Djblets and Review Board unit tests pass.
Implemented a custom paginated response class for use in remote repository

fetching, which needed a different model for pagination (page-based indexes

instead of item-based), and needed to supply slightly different results

(no 'total_results' field). I was able to successfully use it to move back

and forth through results.