Feature/metronome-shortcut

This pull request is focused on the metronome functionality. In particular, the result of this pull request should:

- [x] Add the ability to start/stop the metronome by pressing **?** or **/** key (keyboard shortcuts must be enabled).
- [x] Update the *info* modal dialog so that the shortcut is displayed in the legend (Jeff Martini).
- [x] Fix issue with the metronome in Safari (only plays once). Ensure it works in Chrome, Safari, and FF.
- [x] Refactor metronome into separate view/model components.
- [x] Add unit tests (i.e. jasmine specs).
- [x] Review

@jazahn or @MichaelDHilborn-Harvard  Can you take a look at this when you get a chance?