Remove the need to put ! in privmsg to the bot

[webpigeon]

As users who are PMing the bot already know that they are talking to the bot, it doesn't need to be prefixed. We could remove the need to prefix the commands by:

• Moving the prefix logic into the listener
• Modifying the channel listener to subtract the trigger string (char) from the message before passing it on
• Checking commands to remove references to the trigger if present
• modifying the tests to use this new style

This change would allow us to:

• Make writing commands which work with regexes slightly simpler
• Allow the bot to respond to other formats of commands (nick: ) -- commands don't have to care (they never see it) -- makes rewrite rules/aliases easier (nick: webpigeon++, !webpigeon++ and /msg nick webpigeon++ are all the same command)
• Mirror how supybot/eggdrop users' supply commands

[webpigeon]

CommandListener strips out the trigger so that the processor and plugins don't have to care about it. This can be closed now.