Rework Send(Message|Notice).

They are now commands, with a factory to produce them and no longer exist as services.
Won't need ChannelType yet, Channel shouldn't need mode tracking yet.

(This should be in something 'higher' up)
Renamed IrcUser to UserBase

..and removed the reference to it in ChannelUser.
Cleaned up unused imports
Added basic connection that wraps TcpClient
Client now uses the IConnection