Improvements to docx writer.

Avoid reading image files again when we've already processed them.
ImageSize: Remove no longer exported readImageSize.
pandoc.hs:  Fix writerSourceDirectory when a URL is provided.

It should be the URL up to the path.
ODT, Docx writers:  Properly handle URL refs for images.

These images are now downloaded instead of being ignored (as
used to happen in the docx reader) or causing an error (as
used to happen in the odt reader).