unlike url2png or http2png, NOT storing screenshot but convert any binary data into lossless PNG

1. use 24bit
2. use chunks metadata (zTXt)

• http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html
• http://blog.client9.com/2007/08/19/editing-png-metadata-from-command-line.html
• http://blog.client9.com/2007/08/28/python-pil-and-png-metadata-take-2.html
• http://iamcal.github.io/PNGStore/