Make Pillow the only supported imaging library.

Matplotlib already optionally depends on Pillow (not PIL!), so it's nice
to keep things simple. Plus, there are a few minor differences between
them that will just make supporting both troublesome.
Fix ResourceWarning of Image data.

If a Pillow Image is opened and only the metadata is used, then the
backing file handle is not closed, and must be done explicitly.
Merge pull request #493 from QuLogic/pillow

Make Pillow the only supported imaging library.