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
added meanbean library to project for testing purposes
added Account equivalent factory for testing purposes
added Accounts test factory for testing purposes
added DropURL list test factory for testing purposes
added QblPrimaryKeyPair test factory for testing purposes