set max-width on footer, align to content, buttons equal size

[trebor]

a quick patch to set max-width on the footer to 640px, and equalize the prev & next button sizes.

closes: #188.

looks like this:

[mbostock]

Sorry but this removes the auto margins on the footer, which breaks the footer layout on wide screens.

Before (main):

After (this branch):

I think you want to revert the change to L51–52, but keep everything else.