Add basic Alert component

Hi!

First want to say awesome project!

I made this component today and thought you might want it. 

As far as i can tell it implements all options of [bs alert](http://getbootstrap.com/components/#alerts).

Have also added the additional functionality of calling the `onDismiss` fn after a timeout via the `dismissAfter={5000}` option. It's not typical bootstrap behaviour but is nice to have for the minimal amount of code it requires, let me know if you want it removed/want to stick more closely to the bootstrap default functionality.