bubbleChart: set opacity on circles during render instead of <g> element

I noticed a small bug where r/opacity attributes were being set on the <g> element instead of the circles themselves and I thought it was worth contributing back.  I didn't see any bubbleChart tests in /spec, so I just added a simple vows test instead - hope that's okay.  Please let me know if I need to do anything else, and thanks for a great library!