Added event to notify other grunt plugins that connect has been started.

I have added a grunt.event.emit so that other grunt plugins can be notified of the connect startup. I require this functionality for my grunt-websocket plugin so that it can bind itself to the connect http server so that both websocket and http can be served from one port (80) instead of having a different port for my websocket server. This way it would be more proxy and NAT friendly. I kindly ask you to add this to the main repo as this might be useful for other plugins aswel.