Some updates for the irssi proxy module

I added a few creature comforts to the proxy module.

* Changed the signals slightly so that there is no "proxy client connected" until a user has authenticated with a password
* Made the console output include more useful details and go to the server windows
* Added a toggle to enable / disable the module; on by default to prevent existing user confusion
* Updated documentation
nitpick: spaces instead of tabs for indentation - and yes, it was wrong before, but there's no reason to keep it like that when changing this line.