sml2 now ignores parser errors by default

Minor update to the sml2 conversion script:
It now ignores parser warnings and errors by default.

This allows using the sml2[.exe] program for converting pieces of XML, and not just complete valid files.
For example a piece of XML with namespaces would trigger an error if the namespace definition is not present.

Libxml2 2.9.8 with support for SML

This version of libxml2 can parse both XML and SML, and save either kind.
The sml2 program (sml2.exe in Windows) can convert XML to SML, then back to XML, with no change.

See the README for details.