Added listener to be able to set countdown 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.