MPO Read Support

These changes add the ability for Pillow to read MPO files and extract individual pictures from them.
I don't think so; please note that the JPEG plugin populates the 0xB002 values based upon the size given in 0xB001. This can only ever fail if something is programmatically incorrect within the JPEG plug-in. I made it an assertion because it can be helpful during development but won't consume any cycles within an optimized build.