Atomic reminder

An upgrade to remindme using the US naval observatorys atomic clock as a reference.

As a response to Bobs challenge (http://blogs.mathworks.com/pick/2008/06/27/atomic-clock-alarm-challenge/), I integrated the suggestion by JesseL to use the US Naval Observatory clock as a reference in the original remindme by Matt Fig. Accuracy is limited by lag in retrieving the time, and no corrections are done for clock drift from the alarm is set until it is triggered.