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
fix the radiogroup-spec cleanup
fix the barchart-spec test to test what it says it tests

it still fails, but now it fails testing what it says rather than because it tested something else.
remove merge conflict resolution info
Initial work on SelectOneQuestion widget