convert palettes to hex literals (issue #2)

...also add a gray64 to match Octave's default colormap size of 64 (see http://hg.savannah.gnu.org/hgweb/octave/file/tip/scripts/image/gray.m)

Later I might try having these generated programmatically, and adding the remaining ones from Octave.
fix alpha values
Merge pull request #3 from waldir/patch-1

convert palettes to hex literals (issue #2)
Improve properties and define more utility functions

One key addition is to look for a property called "spatialproperties" which can be set to ensure that algorithms needing to manipulate spatial dimensions can do the right thing.
Implement sliceim
Small I/O fixes
Implement named indexing
Merge branch 'master' of git://github.com/timholy/Images.jl