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.
Unicode, because it's returned in a JSON response. The examples in Riak's docs look like base64-encoded data, so I'd assume ASCII-compatible.