Added countdown finished callback

Added a callback for when the countdown is finished that can be referenced in html using an attribute. This allows you to have a function in your controller's scope fired when the countdown finishes which enables your app to easily take an action when the countdown naturally finishes.

For example, if you had a count down to the start of an online event you could trigger the action that would hide the timer and take the user in to the event once the countdown is finished.