Center index page and remove horizontal scrollbar

The body's non-zero left/right padding (and a non-specified margin,
which most browsers will default to something non-zero) with
`width: 100%` causes the index page to:

- Have a permanent horizontal scrollbar -- the actual computed width
  is greater than the size of the browser window
- Appear slightly off-center on page load -- pushed to the right by the
left margin and padding

This commit leaves the effective body top and bottom spacing as-is (
though it replaces the implicit non-zero margin with zero margin and
padding to make up for what browsers usually set as an implicit body
margin), but removes the left/right padding and margin.

Tested on OS X Chrome/Safari/Firefox and iPhone Mobile Safari.