Added alarm sound when timer reaches 0:00.

- Added Alarm class.
	* new Alarm(audioId, interval) object puts Alarm.ring() on interval.
	* Alarm.ring() plays the audio tag with id audioId.
	* Alarm.stop() sets global alarm var to undefined and clears the interval.
- An Alarm object is instatiated and assigned to the global alarm var when a Time object reaches 0. The alarm is stopped in resetTimer();
	* I'm not happy about the coupling involved with the Alarm and Time classes. I considered making the alarm a property of the Time class, but this had other implications since the alarm needs to play when the Time object reaches 0, but the Time object currently deletes itself when this happens, which in retrospect was a poor choice. I could add an boolean active property to the Time class and set this instead of deleting it, but it doesn't really seem worth it at this point, since this project really isn't that complicated and isn't going to become so. If for whatever reason it does, consider revising this part of the design.
- Added tim_tum.ogg as alarm sound effect.
	* Added tim_tum.ogg to audio/CREDITS.txt
- Added descriptions for playAudio() and resetTimer()