Standardize Handling of Multi-Image Files

This is in response to a Google Groups [suggestion](https://groups.google.com/forum/#!topic/scikit-image/EpU3pfB6SGQ) to use `tifffile` to handle `MultiImage` TIFFs.  `PIL` supports multi-page [GIF](http://effbot.org/imagingbook/format-gif.htm).  

`imread` is now capable of handling multi-image files as an `ndarray`.  `ImageCollection` is now capable of handling multi-image files.