Ordered GroupBy Operator

Hey David, some time ago you created some helpful Transformers in this library that buffers sequential items while they are equal or a condition yields true.

How hard would it be to do the same thing with a `GroupedObservable`?
http://stackoverflow.com/questions/39354421/rxjava-creating-a-groupbyuntilchanged-operator

I imagine it would be helpful to break a large `Observable` source's emissions into finite GroupedObservables based on a condition or a distinct value.