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.
So my feeling here is that returning raw indexes isn't something we'd like to do because the details of indexes are something I'd like people using the models to not have to care about too much. Having compound indexes is something I'd like, but I see those returning parsed tuples of values (i.e. tuples of things that one would set or store on a field).

We're still experimenting with this stuff and I don't want to hold up this PR for this, so I'm letting it go for now. :)