Countdown

Summary:
Addon that allows you to create a live countdown page to
some event.

Here is the ticket that this code is based on
https://secure.phabricator.com/T36

Test Plan:
Tested by manually setting dates in the timer.js file and
checking if they made sense.
I'm not sure if it works across different timezones though.

Reviewers: epriestley

CC:

Differential Revision: 436