added toXML function (inverse of parse), w/ tests

thank you for this, raszi, it's been very useful in a small project i've been working on.  if you're interested, i put together a toXML function to convert jsonml back to xml, with a couple new tests (including a larger file).  having this function makes this lib a little more balanced, and it allows for easier testing on large files, but i'm not sure if you're interested in new features.  either way, thanks again, i appreciate it.