Use relative URLs for images in start.css

I needed this small change so images were shown in the start page. I'm running ethercalc with param --basepath, and expose it to the outside world via a reverse Apache proxy.
Note that I don't have sass installed currently, so I modified the css file by hand, but the result should be the same.

Thanks for the ethercalc, btw! I've long searched for such a software!