Fixes a bug with user events created by an activity module.

Fixes a bug with user events created by an activity module.
The old code assumed that the courseid would always be set but that is only the case for course events, not for user or group events. (See http://moodle.org/mod/forum/discuss.php?d=4466#20827 for a discussion about the meaning of the courseid field in the event table)
I also made the $courseid argument to get_coursemodule_from_instance() optional. It is not needed and in some cases it will not be know, as for example for non-course events created by activity modules.