Added the ability to auto dismiss flash messages

[Emerson]

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 . I also have a hunch that some of the code could be written in a cleaner way.

[jvenator]

Any reason why this was never merged in? Certainly useful, and is a relatively standard feature for flash messages.

[Emerson]

I think the lack of tests was the main reason? As mentioned above, I'm using it in production and haven't had any issues - but having proper tests would give us peace of mind.

Looking at the Files changed tab, I'm also seeing area's where things could be done cleaner and more modularly...

[jvenator]

@Emerson Thanks for the update.

[Emerson]

@jvenator - just a heads up that I recently published an Ember-CLI flash message addon. It has a very similar API to this library and includes the auto dismiss functionality. You can see a demo of it here and the main repo here. Still early days, but would love some feedback!

[jvenator]

@Emerson I'll check it out, although we ended up going with ember-notify for our main app. Thx