[#HLAB-3] Change Metronome Timer

Changed implementation of metronome to use ```setInterval()``` instead of ```setTimeout()``` for more accurate timing. With ```setTimeout()```, the metronome sometimes appeared to "off" by a noticeable amount and wasn't ticking exactly when it should. Although ```setInterval()``` places a higher performance burden on the application, the tradeoff for more accurate "ticks" seems worth it since this is a time-critical function in the app and really can't be fudged very much, which can easily happen with ```setTimeout()```. 

This is to address [#HLAB-3](https://jira.huit.harvard.edu/browse/HLAB-3).

@jazahn and/or @MichaelDHilborn-Harvard  Can you look at this when you get a chance?