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
Merge pull request #209 from k-c13/class-diagram

Update class diagram
Merge pull request #211 from thechauffeur/got-drop-message-from-unknown-module

Handle drop message from unknown module