ported groupByUntil function.
ported groupByUntil function.

samuelgruetter/RxScala,TracyLu/RxScala,ReactiveX/RxScala,samuelgruetter/RxScala,zhongdj/RxScala,hermanbanken/RxScala,hermanbanken/RxScala,zjrstar/RxScala,jbripley/RxScala,jbripley/RxScala,joohnnie/RxScala