Invert coverage by running ytestrunner under istanbul

Unfortunately you need to use the undocumented `--yui` flag for this (so it hooks the YUI loader as well). And I plan to take it out and replace it with something else later.

As long as you are willing to make minor changes to the command line when I fix this, you can accept my pull request.

Otherwise what you are doing seems to be the best option for now.