Fix duplicate NSWorkspaceDidWakeNotification observations

[davidme-stripe]

I incorrectly added an observer to NSWorkspaceDidWakeNotification during the scheduleTimer function. When showing seconds, this means that UTCMenuClock would re-register for the notification every second, and it would re-register observation that many times on each wake. This caused https://github.com/netik/UTCMenuClock/issues/20.

Moving the observation to awakeFromNib should fix it. I also renamed the function to make it clearer that this should be only called once. Sorry for breaking it!