One Bug Fix and One "Feature"

Hi there.

I notice that when you go to interrupt a break timer early, `org-pomodoro-kill` attempts to cancel the currently running clock even when no clock is running.  So one commit tests for `org-clocking-p`.

Also, for those of us snagging this code via MELPA, the "autoload" cookie is convenient.  One less `require' in my .emacs!

Thanks for the code, it's just what I'm looking for!

Trev