Countdown set function and emit 0 as last value

With the listener one can set the countdown value using
$broadcast('countdown-set', value). This might be an edge case, but I
need to set a new countdown. Issue #23 describes how to set the value on
initialization but using this one can change the value whenever
necessary.

Also changed the order of execution of some code to make 0 the last
emitted millis value for a countdown. In the original code the last emit
message was the value just
before 0. For example with a countdown with an interval of 1000 the last
emitted millis value was 1000.