Fix timer so it doesn't start with "9:60"

[mathewsb]

This pulls over the timer fix from AudereNow/audere#666.

Around each minute boundary, the timer has a wonky behavior where it temporarily shows a time like "3:60".

This happens because the current minute is calculated using Math.floor(), while the current second is calculated using Math.round().

I fixed this by first converting to an integer number of seconds. Then the remaining calculations are more straightforward.

I decided to use Math.ceil() here so that the timer has this sequence:

10:00 -> 9:59 -> ... -> 0:02 -> 0:01 -> DONE!

rather than:

9:59 -> 9:58 -> ... -> 0:01 -> 0:00 -> DONE!