Bug 658576 — Need API to get a FolksIndividual from his ID

Add an IndividualAggregator.look_up_individual() method which currently just
looks up in the map from IDs to individuals. In future, however, this method
will also ensure all properties of that individual have been loaded if the
aggregator has otherwise not lazy-loaded properties. (See: bgo#648805.)

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=658576