Replace unicode dependency with ASCII-only equivalent.
example.go now builds, and is more idiomatic
PBM decoding now produces a paletted image.

This reduces a pbm -> png conversion with testdata pbm images from
56kB to 36kB (36% size reduction). If image/png is optimized for
1-bit colormaps, there'll be an additional free reduction to 28kB
(50% size reduction from the original grayscale).
Fixed DecodeConfig not referencing palette in prev commit.
Merge branch 'example'
Merge branch 'palette'