Implement basic functionality
Implement UNO event class factory
Make event subclasses decorate their methods

So the show_exception helper does not have to be called manually.
Refactoring

Make 'not found' errors more generic.
Implement simple database handler

Close #4.
Merge branch 'master' into application-dev
Fix instantiation error
Merge branch 'master' into application-dev
Fix database list result error
Merge branch 'master' into application-dev
Fix errors
Show number of students of every class