Don't append the base url to absolute image urls without protocol.

Prior to this commit the post processor appended the site base url to images with an absolute path that doesn't have a protocol, like images from S3. So the processor converted all S3 images from

    <img src="//bucket.s3.amazonaws.com/uploads/6/4/123.png">

to

    <img src="http://ec2-23-21-129-81.compute-1.amazonaws.com//bucket.s3.amazonaws.com/uploads/6/4/123.png">

on Heroku, which of course broke the image.