New option `org-clock-rounding-minutes'

New option `org-clock-rounding-minutes'

* org-clock.el (org-clock-rounding-minutes): New option to
round the time by N minutes in the past when clocking in or out.
(org-clock-in, org-clock-in-last, org-clock-out): Use the new
option.

* org.el (org-current-time): New optional parameter
`rounding-minutes' to override the use of
`org-time-stamp-rounding-minutes' for rounding.

Thanks to Kevin Buchs for a preliminary patch for this feature.