Another output format for opencv_createsamples

The `opencv_createsamples` tool is already able to create simple training data set from a target image and a collection of backgrounds. But for VisionWorks we need in data set in an appointed format, that differs from the one that is currently produced by `opencv_createsamples` tool. So, this fix adds special key, namely, `-pngoutput` to the `opencv_createsamples` tool, and provides the generating of the data set in a format, that is suitable for VisionWorks.