added new method getKeyIdentifier() in class Identity
Class Contact:
* new field contactOwnerKeyId
* new protected constructor with parameter contactOwnerKeyId
* new getter for contactOwnerKeyId
* new setter for contactOwner
* modified constructor with parameter contactOwner
new class ContactTypeAdapter
added test for Contact de-/serialization
Merge pull request #171 from f-porter/Issue71-Contact-Key-Serialization

Issue71 contact key serialization