PerseusApi: onChangeFocus

Summary:
Adds an `onChangeFocus` method to the perseus API of the renderer
and item-renderer, for better notifications about focus changes.

Test Plan:
 * Tested manually for the whole itemrenderer with the onChangeFocus
   method in test.html
    * tested switching to and between inputs in the question area
    * tested switching to/between inputs in a custom answer area
      (and switching between those and the question are)
    * tested making a single input-number or expression answer-area
      and switching between that and inputs in the question area
    * also tested with expressions
 * Added automated tests for the renderer's part of this api in
   the next diff

Reviewers: charlie, joel

Reviewed By: joel

Differential Revision: http://phabricator.khanacademy.org/D10274