Fix PHP notice in Jetpack_PostImages::from_attachment method

Add logic to cope with images in posts with protocol independent and root relative urls. The URLs returned are always absolute, using WP_HOME if the image src in the post was a root relative URL. This method is used by the Jetpack OpenGraph functions, and the og:image property should be absolute (apparently... though the documentation does not seem to explicitly state this).
We probably shouldn't use the constant here, it would likely be better to use something derived from either `site_url()` or `home_url()` -- as this will cause issues if WordPress is installed in a subdirectory, you'll need to strip off any path component to them.  Or potentially just pull from the $_SERVER superglobal to find out the host?