Banners could do with some accessibility advice?

Banners http://dropbox.github.io/scooter/components.html#banners rely on color alone. It may be good to add a note or similar about making it clear (in wording, or through other means) when something's an error, success message, etc.

(this is something that will be added to upcoming Bootstrap v4 docs - see https://github.com/twbs/bootstrap/blob/v4-dev/docs/components/alerts.md which references/includes https://github.com/twbs/bootstrap/blob/v4-dev/docs/_includes/callout-warning-color-assistive-technologies.md - this change is not in the live version of http://v4-alpha.getbootstrap.com/components/alerts/ yet)