Add a Sphinx extension to copy over Retina images.
Review Request #5511 — Created Feb. 20, 2014 and submitted
Sphinx only actually copies over images that are used in documents. This
means that while we can dump @2x copies of images into _static, they
won't be used.
This extension hooks into the environment generation and html page
collection events, looking for @2x equivalents for all images that are
in use. It then adds them to the list of images set to be copied into
Built the docs and saw the Retina images copied over, which successfully
loaded on the page.