fix #154 – Headers in IE

Fixes #154 and makes headers appear (relatively) consistent across old IEs. 

- `.with-box` divs have a white background on IE8, as it doesn't support rgba()
- `.middle-align` divs err closer to the top on IE8
- image header with filters: filters don't appear at all on IE10 and lower 
- there are still issues with video headers on IE10 and lower, but it doesn't look _totally_ broken