Recursively process images in nested directories

[GoogleCodeExporter]

There are going to be cases where one would like to process images in nested 
directories.

Currently, handing a directory into the `Thumbnails.of(File...)` method will 
cause an exception to be thrown when trying to process a directory, but there 
may be a use case where one would like to process the contents of the directory.

This can lead to some complications with how to output the files, as described 
in the comments for Issue 63.

Original issue reported on code.google.com by [email protected] on 21 Dec 2013 at 3:36