Better summary table

The new summary table layout uses a single table and shows the diff
vertically.  A new row has also been added to show the actual diff in
numbers (blue indicates better diffs; red indicates worse diffs).

Before it used nested tables, which caused problems on Firefox --
overflowed the content area -- probably due to font size differences
(compared to Chromium, where it didn't overflow).

This layout was suggested by Arthur Maciel.