Improve Nikon maker note parsing.

Implement parsing Nikon type 1 maker notes, and don't fail on unfamiliar Nikon maker note formats.

The rename of ImageAdjustment to Nikon_ImageAdjustment is a breaking change for anyone who may have been using that field. But the field value really has to be interpreted in a Nikon-specific way, so it seems to make sense.

(There still seems to be a problem with the Canon maker note parser failing on some images in my library, by which I mean it causes the entire exif parser to return failure, even when it's just failing to recognize the particular format inside the maker note. I'll track that down and send a separate pull request.)