Ability to delete messages programatically after showing them.

Hello, in our project we want to display a growl notification while some content is loading, and if the loading is finished, we want to remove it. I have implemented the according functionality and want to share it with the other users, so could you please merge it in?

If there are any Problems please respond. I had to update the karma version to get the build running, everything else is ok. You can checkout the new functionality on the demo page.