Added initial support for KMZ file parsing and KML GroundOverlay tags

Hi Pavel,

This isn't necessarily a pull request (yet) but more a means of showing you what I've been working on and getting your feedback.  My goal with this project was to support KMZ files which contain images for GroundOverlays.  I have created a L.KMZ class that handles the unzipping (using zip.js) and creates blob:/ references for each references file in the doc.kml contained within the root of the KMZ.  I've added to your L.KML a method called parseGroundOverlay and made a couple other small tweaks to enable it.  I tried to keep the adjustments to your L.KML to an absolute minimum.

I don't have an email to communicate with you outside of this 'pull' request mechanism, but if you have a few moments would appreciate your feedback.  I could see including this L.KMZ within your bigger plug-ins project if you were so interested.

Owen
owenDOTpetersonATgmailDOTcom