Fix display option

The images directive already had a display option, but as was, it only took a single field for the image format. I have reworked this so that now you can pass a triplet: id,format,dpi to the :display: directive and the image displayed in the HTML output will use this spec.

In addition i've reduced the excess space around the image.

There is also a commit included that fixes a bug in sphinxreport-clean that was calling clean.py with main(argv), while main was defined as def main().