Fix Channel.Color interpretation in ImageJ

This prevents the color from being interpreted as ARGB instead of RGBA.
Fake: allow channel color to be specified

The 'color' attribute allows the color to be specified as an RGBA
integer, in either base 10 or 16.
Fake: minor fixes to channel color parsing/population
Merge pull request #767 from melissalinkert/imagej-color

Fix Channel.Color interpretation in ImageJ