Add pngcrush PNG Optimzer

Added support for the pngcrush optimizer.  Really just copied the jpegtran example
Add Option to Disable Etags

I noticed that Tornado is automatically calculating etags for
images, but that didn't work well with my caching strategy.  I have
added the option to disable this.  I have it on by default since
the current version of Thumbor will auto-generate etags for you.
Merge branch 'pngcrush'

Conflicts:
	thumbor/config.py