Viewer.show did not return a value, however ImageShow.show expected
Viewer.show to return a non-falsey value if successful. Therefor ImageShow.show
would continue to call multiple viewers.
In py3k, imports are absolute unless using the "from . import" syntax.
This commit also solves a recursive import between Image, ImageColor, and
ImagePalette by delay-importing ImagePalette in Image.
I'm not too keen on this commit because the syntax is ugly. I might go back
and prefer the prettier "from PIL import".
