data_migration bug fix: include group fields in registration and exercise for maxgroup calculation
update forms, views, templates to use group as foreignkey
import registrations: update student data only if no previous data
enter exercise results: display selected group(s)
query registrations: sum students for each assistent
query registrations: two bugs fixed
export students, import exams: fix empty (None) subject and group fields
query exercise
import exercises: allow comma as decimal point
import exercises: ignore empty lines in csv file
enter exercise results: catch 'exercise already exists' also
    if exercise from different group
Merge branch 'master' of github.com:cwyss/student-manager
enter exercise results: print group choice nicer
import registrations, status report: count unchanged student data separately
non-unique modulo matrikel list filter: use get_pure_query_set() (total_points annotation problem)
save exercise results: exclude inactive students
print exercises: select students from group;
	exclude inactive or without exercises (alternative)
query students: field 'active' included
import exercises: print info for updated exercises, i.e., old and new points
print exercise list: percent column added