ENH: Match pathname pattern to avoid extraneous files.

@tacaswell I pounced on this because I've been meaning to incorporate ``glob`` for awhile.

``ImageSequence('path/to/images/*.png')`` is now the preferred usage. If you specify a directory without a filename pattern, ImageSequence will work the same way as before. But it warns that this could pick up extraneous files.

@RebeccaWPerry, I think it this more sound than skipping over unreadable files as you go. ImageSequence needs to start with an accurate count of the number of images, so it can do fancy indexing. This is also more like the way it's done over at scikit-image.