Move rgb2ntsc and ntsc2rgb to image package

[Rik]

Carnë,

As you are maintainer of the image package, I'm writing to coordinate on
some of of the lesser used image functions in core Octave that should move
to the image package.  This sort of clean-up has already been done for the
statistics functions.

It seems like rgb2ntsc and ntsc2rgb are pretty specific and should be moved
to the image package.  However, the general functions rgb2gray and gray2rgb
which use the luminance channel of rgb2ntsc should be written.  I don't
mind doing that if you can take care of incorporating the other two into
the image package.

Cheers,
Rik