Message should inherit from WrappedMessage

[fs]

Currently, Message and WrappedMessage are completely independent objects. This lead to problems because WrappedMessage? did not have all attributes of Message which some providers relied on.

Furthermore, the interfaces of both classes may be different without good reason (e.g. Messages can send themselves but WrappedMessages can not). I think WrappedMessage should become a super class of Message and all 'basic' functionality should be moved to WrappedMessage.