Qabel/qabel-core#154 Use 'Date' instead of 'long' for DropMessage's timestamp representation.
Also have DropMessage (de)serializers generate/convert these Date objects from/into ISO 8601-formatted UTC timestamps.
Don't use class-global 'json' field for DropMessage serialization tests but use a method local variable instead.
Qabel/qabel-core#154
Added missing test for message time equality in DropMessage serialization test.
Also noticed that a Date has to be truncated.
Dunno whether this may causes any problems as milliseconds will be stripped away once a Date has been transmitted as string timestamp.
Added DropMessage serialization test which checks whether a JSONized DropMessage is an actual JSON string.