Add empty Drop class for sending acknowledge message
Add ErrorObject in Drop as descriped in https://github.com/Qabel/qabel-doc/wiki/Qabel-Client-Drop#special-messages
Add function to create easily a filled drop message
Add function to create an acknowledge id
Add version number to Module
Add functionality to handle Drop message (acknowledge and error object)
Update test for Drop message (acknowledge and error object)
Add ErrorObject
Add acknowledge received to drop callback classes
Check first the identity on AcknowledgeId object
Add send function for one contact and implement both  send function (not
finished)
Update retrieve() function
Update test