provide temporal alerts

Sometimes, something significant is happening that we want to call attention to, like "the location for this week's hack night is different", or "don't forget to register for the upcoming hackathon", or "please comment on Boston's Open Data Policy on website.com".

These should be displayed above the fold, perhaps on every page, perhaps on just the homepage. They should probably be somewhat automated, as we'd want them to disappear without human interaction after they're no longer relevant, however it'd be nice if they're fairly customizable too (e.g., including custom markup).