Change: use fake timers in tests.

Test suite can be speed up significantly by using fake timers provided
by the sinon library. See http://sinonjs.org/docs/#clock-api and
http://cjohansen.no/en/javascript/faking_timers_and_dates_with_sinon for
more information.
Merge pull request #22 from nisaacson/fake_test_timers

Change: use fake timers in tests.