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).
WordPress traditionally doesn't like protocol agnostic links, I'd suggest wrapping this in a `set_url_scheme()` call to set it to whatever the current request is, http or https.