Five changes:

1) When sizing an image, default to the actual size, not 0.
2) Handle strings (in CSS) containing non-ASCII characters.
3) More robust handling of file not found.
4) Extent the rgb command to also hangle the rgba command.  The fourth
   parameter (transparency) is ignored.
5) In handling the TD tag, if the TD node has no children, it there is
   nothing external to the node to define the width of the column.  Set
   the width to the sum of the right and left margin.