filenames can now contain slashes - this allows the use of subdirectories to store images

This 2-character change allows one to use a hierarchy of subdirectories to store images in, so that you don't have to put too many images in one directory. For instance, see this Stack Overflow question:

http://stackoverflow.com/questions/12256835/folder-structure-for-storing-millions-of-images

I did not add a test, but all the tests currently run. Let me know if you need a test for this.