New timer functionality to set a timer from a headline.

New timer functionality to set a timer from a headline.

This commit defines three new functions in org-timer.el:

- org-timer-set-timer, bound to `C-c C-x ;' in Org buffers
  and to `;' in Org agenda buffers.   This function sets a
  timer for the headline the cursor is currently it.  Up to
  three timers can be set.

- org-timer-show-remaining-time: this shows the remaining
  time for the last set timer.

- org-timer-reset-timers: this reset all timers.

This functionality was requested by Samuel Wales and emulates
that of tea-time.el -- see the emacswiki doc about tea-time.el:

  http://www.emacswiki.org/emacs/tea-time