Add tests that show how cross-class comparison works

I wanted to document the behavior of using attr_comparable across classes. In particular a reminder that you probably want to include `:class` as the first attribute being compared. I like that attr_comparable doesn't assume you mean to scope comparison to instances of the same class, but I believe its is the most common use case.

I'd like to add this morsel of info to the README as well. Should I add another example? or could I augment the existing one.


Also, it might be a good idea to attach a license to this repo one way or another. I'd recommend MIT.