Since itex's \begin{svg}...\end{svg} syntax allows
the client to pass arbitrary junk through the document,
we need to check that the result is well-formed.
Use a pluggable XML parser: nokogiri, if installed,
REXML otherwise.
Add a Rack Metal itex endpoint.
Add an itex tool to SVG-Edit.
Disable the foreignObject tool
(at least, for now) as it doesn't
currently play nice with the itex tool.