In issue #106 @artfree mentioned that the new XMP functionality can slow things up.
So it's better to make it optional, and turn it off by default.

You now have to call EXIF.enableXmp()` before XMP is retrieved. Sadly I couldn't find an example image with XMP info, so I couldn't really test it. But I checked that indeed the call wasn't made.

In the course of this issue I also moved the example page to the root, and added the missing xml2json function as suggested in other issues (#84 and #92).