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.
We could, but it's an opaque thing we get from the client library and give back to the client library. It's guaranteed to be JSON-safe because Riak returns it in JSON, but it may be `None` if there's no continuation.

I don't really want to make unnecessary assertions about the types of objects things we get from third-party code because it makes our tests fragile.