Changed gallery layout: Fluid square thumbnails.

Hello nomnomnom,

a pull request, that doesn't fix anything. I changed some things - judge for yourself if you like them ... :)

I thought it would be nice to have square thumbnails filling the viewport without leaving blank space. Additionally I added media queries to display a different number of thumbnails in a row depending on the viewport size.

If you like the changes I'd happy if you pull them in.

Best,
chrK