0.15.0

• Added Flowables.intervalBackpressure(): interval with virtual buffering in case of backpressure (Example).
• Added FlowableTransformers.cacheLast(): cache and emit the very last item. (Example)
• Added FlowableTransformers.timeoutLast(): emit the latest if no new item arrives within a timeout period. (Example)
• Added FlowableTransformers.timeoutLastAbsolute(): emit the latest when the source completes or the timeout happens. (Example)
• Added FlowableTransformers.debounceFirst(): debounce variant that takes the first item in the window and then drops items until there is no item within the specified time window relative to the latest item. (Example)
• Added FlowableTransformer.switchFlatMap(): a combination of switchMap and flatMap: flattens up to a maximum number of mapped inner sources but replaces the oldest one if a newer inner source arrives beyond the active limit. (Example)