Adding :scope option to previous/next methods (and rails4/ruby2 support from fny)

As I mentioned to you on IRC, here's at least something in terms of being able to have next/previous work over a given scope.  I was wondering what you think about having a by_star_scope accessor on a model - either a block or a variable that would return a scope Arel query to use as the basis for the next/previous methods.

Thanks!