Fixes case when joining an unjoinable irc channel

Missed a case when a channel is joinable. Plugin now properly exits.
Fix sending messages to protected irc channels.
More consistent use of variables.
Improve docs on protected irc channel password

Also fixed an inaccurate comment.
Fix joining with password in other locations
Merge pull request #1036 from ecnahc515/fix_irc_join

Fix sending messages to protected irc channels