added de-/serialization of multi sub-keys of QblPrimaryKeyPair
added de-/serialization of multi sub-keys of QblPrimaryPublicKey
added multi sub-key support to class Contact:
- new getter for retrieving list of sub-keys
- marked old getter @Deprecated
- changed name of sub-key setters from setX to addX
changed modifiers of generateEncKeyPair() and generateSignKeyPair() to public
changed contactTest() for testing multi sub-key serialization
changed syncedSettingsTest() for testing multi sub-key serialization
Removed generic parameter from DropSerializer
Removed redundant generic parameters from test DropMessageGsonTest methods
Introduced as()-method in ModelObject to syntactically work-around casts e.g. when using DropMessage.data objects
Fixed file en/decryption test tear-down and test interdependency

Closes #221
Closes #222

Test files are now removed after testing and the decryption test is now
longer depending on a pre-running file encryption test.
Update class diagram
Merge pull request #220 from aBothe/genericremoval

[Drop] Stripped generic parameter from DropSerializer
Merge pull request #219 from f-porter/Issue-206-add-multi-sub-key-serialization

Issue 206 add multi sub key serialization
Merge pull request #224 from k-c13/class-diagram

Update class diagram
Merge pull request #223 from cburkert/fix_tidyup_test_files

Fixed file en/decryption test tear-down and test interdependency