Added the ability to auto dismiss flash messages

This pull request adds the auto dismissing feature outlined in issue #16. It's a little rough around the edges, so I totally understand if we don't want to merge it in yet, but I wanted to get it on the radar.

I'm using this code in my app and everything has been smooth so far, but the fact that I was not able to write a proper test for this makes me sad :rage1: . I also have a hunch that some of the code could be written in a cleaner way.