Migrated MultipleChoiceQuestion and MultiSelectQuestion (including demo file and dependencies) to Closure

MultipleChoiceQuestion and MultiSelectQuestion and their demo files is running on Closure (OOP-sified and compiled).

All their dependencies were migrated to Closure (and compiled) as well
* js/widget-multiplechoicequestion.js
* js/answerman.js
* js/submitmanager.js
* js/widget-radiogroup.js
* js/widget-button.js
* js/fakeactivitydb.js (In the real release, we will remove this from compilation)
* js/widget-multiselectquestion.js
* js/widget-checkgroup.js
* js/answermanpolling.js

Other affected files:
* [modified] js/build.sh
* [modified] multiChoiceTests.html
* [added] multiSelectTests.html