Example groups can override the default ordering

So ... is this even a good solution and good way to design it? :smile:

There's a decent bit of weirdness around how ordering works right now
that we're trying not to break. If you see some weirdness in our code,
feel free to ask about it ... and we might decide to simply break it for
RSpec 3 to allow the solution to be cleaner.

Still outstanding if so:
* `OrderingRegistry` has some structural duplication between
  getting/setting ordering for examples vs. groups
* We have not yet written rdoc/yard for the public interface for
  `OrderingRegistry`. Some of these methods might be `@private`.

@samphippen and I paired on this