Added DropURL class
Removed checks and tests of drop url syntax form DropServer

These tests are specific to URL fully qualifying a drop instead
of a drop server.
Therefore, the checks and tests have been moved to new drop url class.
Adapted code to use DropURL class
Merge pull request #158 from cburkert/dropurl

Introduction of DropURL class
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
Added HMAC and GCM en- and decryption
added encryption functions with nonce parameter
ctr mode test vectors are working
gcm mode test vectors are working
removed unused imports
added handling of nonce=null and counter.length=0
finally added second nonce=null handling
added comments for code understanding
Merge pull request #146 from roeslpa/apply_test_vectors

GCM and CTR mode with 96 bit nonce