Bundling: added <folder> element support

Loads all the files in a given folder (optionally including all sub-folders).  Useful for AMD type scenarios where the file order doesn't matter and you don't want to have to keep adding new files into your bundle definition.

Also took a stab at updating the xsd file, which you can find at http://upta.me/bundle-with-folder.xsd (first time I've done anything with xsd, so hopefully it doesn't suck too bad ;)