renamed multiple choice widget

I decided that I really didn't like the name SelectOneQuestion, and although the name itself doesn't include it, after discussiong it w/ Leslie, I decided that the common meaning of a multiple choice question is select one, so I went with that.

I've also cleaned up a few more things, added comments that were missing/incorrect.

@lbondaryk I still lean towards the RadioGroup and the SelectGroup firing selection events and not value changed events. I have not yet changed the SelectGroup to do that though, and it needs to in order to work in the MultipleChoiceQuestion (see multiChoiceTests.html example).

I think the functionality for ECOURSES-145 & ECOURSES-130 is done, however for the stories to be done I really need to implement some unit tests of the MultipleChoiceQuestion widget, and possibly also of the changes I made to the SubmitManager, although that needs some cleanup 1st.