Removed usage of model class Contacts from drop interface

According to the model-view-controller concept, the Contacts class
should not be exposed in the drop interface.
Instead, a collection of "Contact"s is used.
Merge pull request #151 from cburkert/fix_mvc_incompliant_usage_of_contacts

Removed usage of model class Contacts from drop interface.
Fix #111 Drop::sendAndForget can return a false value
Move DropResult and DropResultPair to a new class
Update DropMessage<T> to DropMessage<? extends ModelObject>
Move DropResult and DropResultPair to a new class
Update DropMessage<T> to DropMessage<? extends ModelObject>
Update DropMessage<T> to DropMessage<? extends ModelObject>