WIP: update directory structure, use polymer 0.10

If we take this change, it will require lots of changes to text and (I think) images in the Dart Tutorials. (At the very least, we'll need notes about the directory changes.)

I think the httpserver code/text needs a bit of tweaking, to make sure the URLs you're telling people to use actually work and have files in the right place.

Also, the polymer examples need to have the version updated, once polymer 0.10 is out. It should probably be:

    polymer: ">=0.10.0 <0.11.0"