Accept setting of maxgray when creating PBM images

Explicitly (but unnecessarily) setting maxgray to 1 when creating
new images from bilevel data is now accepted for PBM images and does
not trigger the data being interpreted as PGM grayscale pixel values.
Only maxgray values of 2 or greater imply a PGM image.
Improve documentation of pixel value interpretation

Clarify the mapping between pixel value and corresponding color:
for PBM the value 0 signifies white and for PGM/PPM it signifies black.
Release 0.3.1