Merge pull request #19 from ccgn/master

Abstract the in memory representation of an image and implement more met...
Merge pull request #20 from ccgn/master

Quick fix: add mod pixel to lib.rs
Added png test suite

Adds the png test suite and some initial tests.
Prevent failure on strange chunk types

The chunk type is not considered to be a valid string.
Merge pull request #22 from nwin/master

Prevent failure on strange chunk types
Merge pull request #21 from nwin/add-png-testsuite

Added png test suite
Merge remote-tracking branch 'upstream/master'