added a 'timer-set-startTime' listener

added a watcher to listen on 'timer-set-startTime' events. Before there was the problem that when you used the start-time attribute the value could not be changed after compilation. you had to compile the whole timer object again. this method should fix this problem by setting the startTimeAttr manually

#88 is a similar problem. you postet the plunkr using $compile but that is not realy the nice way is it? ;)
#44 is a similiar problem as well. in that case the problem was the countdown value.

if you find another way to handle this problem please tell me