Added channel and channel user objects to the server object.
Keep a list of servers on the main window.  Create a function to generate the tree from the servers/channels.
Changed the flow of the application:  message_parser has a reference to the server, so it is able to add and remove channels or channel users, update text, etc.  The irc_server and irc_channel emit parsed_message signals when their text is updated, so we can update the main text, tree view, or user list.
* New irc_channel instance added to server when typing /join
* Refresh tree when joining channel
* Fixed server tree only showing one channel
* Separated /join command from rest of parseCommand function to allow multiple types of commands
Emit channel message changes to main window.