skeleton for CourseModule; ActivityCollection
remove selectedGroup

on a second thought, selectedGroup as a property of CourseModule domain
level entity doesn't necessarily make that much sense
rename ActivityCollection -> ActivityGroup

we're not implementing Collection, aren't we
some class fields now private
some javadoc (on the purpose of ActivityGroup)
even more javadoc for ActivityGroup
a basic test for activityGroup
CourseModule: toString method, (empty) test class
progress diary update strikes again!
Merge branch 'course-module-implementation'

...well, at least CourseModule has now cosntructors and stuff. In
retrospect this didn't warrant a topic branch of its own.