Add git attributes file for *.ppm, fixes #92
Revert "Fix KeyError in Tests/test_file_xpm.py"

This reverts commit b29326b17534f5fec04c0bec35c2f1eff6499f73.
Revert "Added support for PNG images with transparency palette"

This reverts commit 5baa1ac1b8d41fcedce7b12ed1c4a8e87b4851bc.
backport PIL's PNG/Zip improvements

- add new FASTOCTREE quantizer with alpha support
- make ZIP compress level and type configurable
- support reading/writing PNGs with paletted alpha

source https://bitbucket.org/effbot/pil-2009-raclette/commits/3637439d51693c92028a7c4cfa75f5db583de357
release GIL during Image.fill
reduce PNG palette size for images <255 colors

limit color and trancparency palette to 2^bits entries, when a PNG is
saved with the 'bits' PNG encoder option
Merge pull request #94 from olt/gil_image_fill

Release GIL during Image.fill
Merge pull request #95 from olt/pil-png-transp-backport

Backport PNG/ZIP improvements from PIL repo