Support lenient reading mode

Lenient mode warns instead of raising exceptions when
checksum validation fails. Useful for working with
malformed but recoverable PNGs.
remove defaults for gen
Merge remote-tracking branch 'lumberlabs/lenient-support'
tweak doc for compression
return palette in info
more palette tests
Optimize the (common?) case where the user requests the same bitdepth of the image. With this optimization the time to read a 800x800 image with no filters when from 4.7s to 0.8s.
Add tests to cover the filter and unfilter methods.
Merge branch 'perf' into cython
Create a cython module to improve the speed of some key operations.
Fix a few bugs discovered with png2 test suite.
Complete the cython auxiliary functions
Remove lingering print, and make more cython functions nogil.
tests now run after merging pull request
Integrate my unit tests into png unit tests.
Remove obsolete test.
Integrate pngfilters in png.py.
Merge pull request #9 from e98cuenc/cython

Speed up png reading with (optional) Cython
run 2to3 by hand if we think necessary
Merge branch 'master' of github.com:drj11/pypng