GroupBy with element selector

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We are missing the groupby overload with element selector http://msdn.microsoft.com/en-us/library/bb534304(v=vs.100).aspx. See here for a use case vigdorchik/rxmon@db31749.

If we pass in an element selector we can make this much more efficient https://github.com/Netflix/RxJava/blob/master/rxjava-core/src/main/java/rx/internal/operators/OperatorGroupBy.java.