Event observers

HI, David. I have rebased version of subcourse module with event observers on your latest master. It does not need manual grades fetching user action at all, so teachers allways see actual grades and 
grades, and module activity link leads directly to referenced course.  Cron fetching action is needed just to иу shure, that old grades are synced, so it's configured to run once a week by default.

Please take a look.