plugin-clock: run-at-time every min with 0 sec

> if time is t, then the timer runs whenever the time is a multiple of
> repeat seconds after the epoch. This is useful for functions like
> display-time.
>
> http://www.gnu.org/software/emacs/manual/html_node/elisp/Timers.html
Merge pull request #78 from nishi-maki/clock

plugin-clock: run-at-time every min with 0 sec