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
Added "open" as a supported image viewer (for OS X)
Remove spaces internally from imclass

ImageMagick seems to report the image class differently on different
platforms. See #4.
Improve README
Add iterator.jl (not yet fast) and test/readremote.jl
Add interface for 1D iteration
Require that images be StridedArrays, not AbstractArrays
Avoid immutable iterators for now and just use integer indexing

This isn't nearly so pretty, but at the moment the nicer solution
is not getting the needed performance due to limitations in inlining.
Add dispatch-based scaling/clipping/type conversion
Add display via Cairo
Update README
Fix widthheight
Steps towards resize support (doesn't work yet)
Support display in a pre-allocated WindowImage buffer