PNG file support

[zuhito]

Human-Pose-Estimator has the same problem as Facial Age Estimator. Image Caption Generator, Facial Recognizer, and Object Detector support both JPG and PNG files. On the other hand, Human-Pose-Estimator and Facial Age Estimator can handle only a JPG file. It will be easy for developers to switch MAX container which is related to image recognition if all containers support both JPG and PNG files.

[MLnick]

I think this is best handled at the level of MAX-Framework to provide common utility functions that can be re-used by all relevant models. See IBM/MAX-Framework#8