add playground

[jacekkopecky]

This example wants another page, playground.html, that has a drop target that is a cat playground - then we could drag and drop between the two pages, even in different browsers.

for that to work reliably, dragParent in #1 will want to be cleaned up when a drag is completed or cancelled. If we don't, and we drag a cat from clowder to vet, an immediately-following drag from a playground to clowder would likely fail.