FontFolders - added enumerateFonts and sanity check for file existence.

On Mac OS X I was getting this error when running buildCache:

    /usr/share/fonts: openBinaryFile: does not exist (No such file or directory)

I think it was occurring because buildCache would try to open a non-existent folder as a binary file, thinking it was a font file. I added a check for file existence.

I also added a simple utility function for listing the fonts found and stored in the cache.