Make timer-start event restart countdown timer

The countdown timer was not starting from the countdownattr value after doing start->(wait >1s)->stop->start, so I added the initialisation of the countdown in the .start function. 

It's a good question whether the line should be abstracted into a function.

Another thing to consider is whether to use a simpler way to determine if the argument is a number. Assuming angular isNumber doesn't do the trick, see a plain-js option here:
http://stackoverflow.com/questions/18082/validate-numbers-in-javascript-isnumeric

I'm using angular-timer for http://worldmap.io (work-in-progress) if you want to see the use case

Thanks for an awesome little directive!