Add XSL stylesheet to fix up SVG files

Commit 29046c44f36099f4c979b1491fcf27db2f9184f9 manually edited the SVG
file generated by Ditaa, which was kind of what we wanted to avoid
having to do.  Here is an automated way to take care of the same thing,
by using an XSLT stylesheet.  This also takes care of another smaller
issue in the SVG generated by Graphviz.

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Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services