Add pagination and return_terms support to Riak index queries

This builds on #845 and adds support for pagination and the `return_terms` parameter to the existing model and manager implementations.

The idea is to do this with minimal overhead. If it turns into a rabbit hole full of yaks, we'll need to find a different plan.
Probably, but `return_terms` lets you do more interesting client-side filtering and such as well and it's easier to reason about than relying on paginated results being sorted while unpaginated results aren't, etc.