Fix fullscreen images.

[bmschmidt]

I'm having some trouble with the data-background-image right now--revealjs seems to be losing it on some nested slides. I've removed it for the time being. Restoration will involve:

1. Standalone image slides are always full-screen with text.
2. Ability to specify text color via a class attribute as 'white', 'red', 'blue', or 'black.' (Those four colors should be enough. Maybe add 'cream' just in case.
3. (Ideally, but hard) Some css class that targets the text at a quadrant; so that you can make sure someone's name doesn't cover their eyes.