No longer possible to have multiple Fancy dropdowns open at once

Summary:
Unlike HTML dropdowns, the Fancy dropdowns weren't closing when you clicked elsewhere on
the page. Now, they listen for a global click on the $(document) and close if the click isn't
within the widget. (Here be minor dragons!)

Test Plan:
- Run Perseus.
- Open test.html (the starting widget now has two dropdowns!).
- Try:
    - Opening one dropdown and clicking elsewhere on the page. The dropdown should close.
    - Opening one dropdown and clicking on the other dropdown. The initial dropdown should close.
    - Opening one dropdown and clicking on the same dropdown. The dropdown should close.
    - Opening one dropdown and clicking on one of its options. The dropdown should close.

Reviewers: jack

Reviewed By: jack

Subscribers: elizabethlin

Differential Revision: http://phabricator.khanacademy.org/D11088