change 'ghostly' effect to a pulsing animation, fixes #8

[sambrenner]

You might want to tweak the animation timing a bit to your liking but here you go. I think this looks quite nice, especially when there's a mix of overlapping and non-overlapping ghostly cells.

[jrheard]

Thanks! It might be up to a week before I get this merged - I'd like to experiment with adding some cljs code that makes this CSS only get applied to ghostly cells that overlap non-ghostly cells, and I'm on a trip for the next few days - but I really appreciate this PR and look forward to playing around with it!