Issue71 contact key serialization

Closes #71. Please review.
If we have wrapper classes like "Identities" to collect identities we should develop the methods to access these identities agnostic to the actual internal implementation of the wrapper classes.

This means it is not a good idea to return a Map because every caller has to change his code. You did this e.g. for the DropTest class. The DropTest class should not need to know the type of the collection.

You just triggered the case why this is unwanted - you had to modify code from another package because of internal changes in this class.