Countdown: Reworked countdown.

 * Removed the option to set delay time.
 * Countdown now automatically changes delay time depending
   on time left.
 * Fixed a typo in docstring.

Signed-off-by: Jakub Novak <jakub.novak@gmail.com>
@pepol the helper function could figure out whether we are talking about minutes, hours or seconds easily, thus taking care of formatting the message altogether.

There is no need to send the message right away but if @JakubNvk wanted he could simply use a decorator.