Fluid notifications (to support Fluid, http://fluidapp.com)

This PR contains additions to the Notifications plugin in order to support the custom notification system provided by Fluid.  Desktop notifications do not work within Fluid, so are not enabled/displayed.  Fluid notifications do not work in browsers so are not enabled/displayed.

This includes:

 * Growl notification integration (as an alternative to desktop notifications)
 * Unread message count as badge on application icon in Dock
 * Bounce for attention on incoming message

Feedback and suggestions for improvements welcomed - this was very much written as an enhancement to support users in my organisation, but I thought it was worthwhile offering up the functionality to the community too!