Pretty alerts

Feel like @bcipolli sending in a PR when I'm supposed to be paying attention :+1: 

This should make users and @wangguan59 a little happier with our alerts - now they are styled much prettier (bootstrap) and have a little x in the corner that you can use to make them go away! 

To implement this, I just extracted the bootstrap styling that applies to alerts into it's own css file called bootstrap-alerts.css, and included that globally. Used this strategy to avoid screwing other styling up by globally including bootstrap on distributed server. Hope that's cool. Please pull and test, see if anything looks funky.