MPO Read Support

These changes add the ability for Pillow to read MPO files and extract individual pictures from them.
It's not just those two, it's that whole document. I used PIL there because that's in keeping with the rest of the document. There are over 100 occurrences of "PIL" in the document, and some of them are valid, so it isn't a "replace all" operation. I don't mind changing all the ones that need changing if we've got some guidelines on when to use each.