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.
Agreed, but that requires a bunch of changes to how we handle indexed fields. I think `return_terms` is sufficiently useful as-is that I'd rather not wait for that to happen.