Modules that perceive commands should properly set terminate

[richteer]

The self.terminate flag in the XMPPModule class should be set to True when the same message should not be passed to any more modules.

And example of this is the Quotes module, where it perceives the command !quote. Other modules probably won't need to receive this, so setting self.terminate = True after the command is handled prevents any other module from receiving this message, such as Haiku or Irc.