Performance fix. Debounce the update method on the roster.
Don't add test users alphabetically, but rather randomly
More optimizations.

Don't sort the global ContactRosters collection (we only need to sort the
individual groups).
Only add the roster to the DOM once the users have been added.
Minimize debounce time for update method to 100ms.
Add another profiling method for adding users without groups
Performance fix. Set display "none" on the roster element.
Some refactoring. Add showInRoster method

which checks if a contact should appear in the roster (depends on
show_only_online_users setting)
bugfix in positionGroup.

Index must always be 0 if there aren't any group elements in the dom yet.