Fix issue http://code.google.com/p/dygraphs/issues/detail?id=58 using plug-ins

Things to pay special attention to:
1. the TODOs in unzoom.js. I'd like those fixed prior to completion
2. The automated test. It's a simple test, so don't fret. :)
3. The new doc type.
4. This is going to require people to copy and install unzoom.js on their own. Do you want to move unzoom.js to, say, contrib/plugins or something?