Changed the `Thumbnails` builder interface to support use of iterables for input and output.
Changed the `Thumbnails` builder interface to support use of iterables for input and output.

* Added a method to return a `Iterable<BufferedImage>`
* Added use of `Iterable<File>` for `asFile`/`toFile`
* Changed javadocs to reflect that the restrictions of input for certain output methods have been lifted.

guhb/thumbnailator,giflw/thumbnailator,qingsong-xu/thumbnailator,ymero/thumbnailator,coobird/thumbnailator