oiiotool --resize/--resample WIDTHx0 or 0xHEIGHT

A '0' value for either dimension will be interpreted to mean "automatically
compute the missing dimension to preserve the aspect ratio of the original
image."  So, for example, if input.tif is 1024x768, then
         oiiotool input.tif --resize 512x0 -o output.tif
will result in a 512x384 image.

Fixes #797