changed saveGray to handle odd behavior of convert.

png files apparantly do not support dpi, only centimeters. Imagemagick's
convert behaves strangely. Documented here.
http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=10201

This hack saves png files with dpc of 30, corresponding to dpi of
approx. 75. 75 was chosen because it is the minimum dpi Nuance OCR
supports. Obviously this is trickery on our part. A higher figure will
work but sometimes higher levels cause failure in practice.