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).
Pedantism on my part, but could you add spaces within your `empty()` function calls so they look more like `empty( $src['scheme'] )` so as to keep with core coding standards?

https://make.wordpress.org/core/handbook/coding-standards/php/#space-usage