Multi selectable Multiple Choice question

Implemented the CheckGroup (js/widget-checkgroup.js) and MultiSelectMultipleChoiceQuestion (js/widget-multiselectmultiplechoicequestion.js)
* I know the name is super long, suggestion are welcome **

Those two classes are analogous to: RadioGroup and MultipleChoseQuestion, with the following differences:
- It is possible to select (check) multiple items
- Max selects is configurable (when max reached, it pushes an event)

Demo file in /PrototypeForm/submitTests-multisel.html

Unit test is still pending.