Throttle rendering of the preview.

Summary:
Now typing in the editor is reponsive again!

- Call ItemRenderer.render() once, instead of once per hint.
- Throttle rendering to once per 50ms, instead of once per keystroke.

Test Plan:
locally, Chrome/OS X
- mash keys in one of the hint boxes in /test.html; see hint update
- type slowly in one of the hint boxes in /test.html; see no keystrokes dropped

Reviewers: alpert

Reviewed By: alpert

CC: eater

Differential Revision: http://phabricator.khanacademy.org/D2132