Update class diagram
Fixes #155 Handle unknown drop message types
Added test case to handle unknown drop messages

"Unknown" means types of drop message no one registered to listen for.
Removed ununsed imports
Fixed bug in DropController.retrieve() where a special state led to a drop message be handled twice or more times. #200
Merge pull request #209 from k-c13/class-diagram

Update class diagram
Handle possible RuntimeExceptions that are likely to be caused by invalid Json syntax.
If this wasn't handled, consciously injected errornous JSON could crash the entire client.
Corrected DropController.decryptDrop() javadoc
Introduced deserialization tests for invalid/illegal JSON syntax in drop messages
Added rationale comment to DropController.retrieve() why InvalidKeyExceptions won't be handled
Pass DropController.sendAndForget() exception to logger for better stacktrace analysis
Pass exception in DropController.handleDrop() to logger
Merge pull request #211 from thechauffeur/got-drop-message-from-unknown-module

Handle drop message from unknown module
Changed bit constants to byte constants
Renamed SYMM_ALT_TRANSFORMATION to SYMM_GCM_TRANSFORMATION
Throw InvalidKeyException instead of catch
Added and improved logging for catched exceptions
Removed unneeded re-initializations of gcmCipher
Added Javadoc about possible null return values
Added file/stream based en-/decrypt methods w/ tests