[errai-users] ErraiBus trunk *will* break stuff.

I have implemented some long overdue refactorings that were getting in the way of API cleanliness. For instance, CommandMessage and ConversationMessage are now constituents of the Message interface. As such, all of the other interfaces such as MessageCallback, now depend on the Message interface as opposed to CommandMessage. This is really no more than a search & replace type fix. But I think it was ultimately necessary for some of things I'm trying to do with the bus. As the API is growing, it was becoming eminently clear to me that the class structure I had was very limited. :)