Implemented call to create user in CommandReceiver.
Post method calls Create on repository if it does not find the user in the system.
Post method on CommandReceiver now calls User.AddMessage.
Created interface IUserWall to represent user wall operations.
Renamed IUserWall to IWall.
Reanamed AddMessage in IWall to Post.
Renamed CommandReceiver to MessageBroker.
Implemented post to user followers on message broker.
Implemented User followers on User.