contrib: improve optimize-pngs.py
contrib: improve optimize-pngs.py

- Check that image contents match pre- and post- crushing.
- Also remove use of external tool to compute sha256 in favor of hashlib.
- contrib: remove all use of shell=True in strip_pngs.py
  Using `shell=True` can be a security hazard. See e.g.
  https://docs.python.org/2/library/subprocess.html#subprocess.check_output

aspaas/ion,MinuteManReserve/mmr-core,aspaas/ion,MinuteManReserve/mmr-core,aspaas/ion,MinuteManReserve/mmr-core,aspaas/ion,aspaas/ion,aspaas/ion,MinuteManReserve/mmr-core,MinuteManReserve/mmr-core