Add cleverbot module

I wrote a Cleverbot module for kicks and thought I'd add it to willie-extras. Works quite well although it uses one Cleverbot session for everyone, so it might temporarily get conversational messages mixed up. If viable, I'll let each user have their own Cleverbot session later on.

Has 4 trigger commands and a regex match:

    cleverbot_think.commands = ['cb', 'cbot', 'cleverbot', 'think']
    cleverbot_think.rule = '[Cc]leverbot[,:]? ()(.*)'