Notify users when time limit is reached

[KShivendu]

After the time limit has been setup, it's important for us to notify the user when the limit has been reached. Since our dashboard is web based, but our tracker runs on client's os. We have the option to either use web based push notifications OR just use the client's background service to fire up notifications using the OSes default notification daemon.

Depends on #11 and #12

Needs discussion with @supercoww

[ambarvm]

We can do this on the desktop client with https://www.npmjs.com/package/node-notifier