Add "offset" option to adjust the time difference between "now" and "end time". 

Add “offset” property to options that adds a period of time (in milliseconds) to the calculation of time difference between *now* and *end time*. Useful if countdown calculation to *end time* is imprecise due to user's date and time settings. Using the (client time - server time difference = offset) calculation a precise countdown can be set for multiple clients with different time settings.

Add “updateOffset” public method that updates the offset.