RadioGroup now supports selectedItem, selectItemAtIndex and fires a selected event.
Updated readme w/ info on configuring karma on linux
Merge pull request #46 from mlippert/widget-refactor

enhance RadioGroup
change draw to redraw to match update in barchart widget
multiple choice dropdown (SelectGroup)

* added test page to book and toc
* used new control in PhysicsResistance example
* added control widget, not question