Supervises IRC client processes

Usage:

# Start the supervisor (started automatically when ExIrc is run as an application)
ExIrc.start_link

# Start a new IRC client
{:ok, client} = ExIrc.start_client!

# Connect to an IRC server
ExIrc.Client.connect! client, "localhost", 6667

# Logon
ExIrc.Client.logon client, "password", "nick", "user", "name"

# Join a channel (password is optional)
ExIrc.Client.join client, "#channel", "password"

# Send a message
ExIrc.Client.msg client, :privmsg, "#channel", "Hello world!"

# Quit (message is optional)
ExIrc.Client.quit client, "message"

# Stop and close the client connection
ExIrc.Client.stop! client