You wouldn't believe what this commit put inside a widget

Summary:
Adds `Group` widgets, which group other widgets together
for the purposes of grading. This widget basically
consists of a renderer.

Test Plan:
 * Overall:
    * Make a group widget
    * Make an input-number in it
    * Try grading for different values
 * Mobile API:
    * Make an input-number outside the group
    * try switching focus between the inputs
    * verify that the onFocusChange results in the console make sense
    * set test.html's staticRender to true
    * call itemRenderer.focusInputPath on the command line and see
      that the onFocusChange output makes sense
    * call itemRenderer.setInputValue and see that the input inside
      the group has the new value
    * I didn't get getAcceptableFormatsForPath() or blur()
    * call itemRenderer.getInputPaths() and verify the result

Reviewers: emily

Reviewed By: emily

Subscribers: dylan

Differential Revision: http://phabricator.khanacademy.org/D13562