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.
If we keep `Manager.index_keys` the way it is now, we should change this to document the different thing that is returnedd when `return_terms` is true.