Width attribute optional in table and textblock

The width is taken from the surrounding area if
it is not given in the attribute.
XPath: clear handling of comparison with undefined

This should be close to the XPath standard
Error handling with Message

Only print a message if the xpath expression results
in a meaningful output.
Release version 1.6
Update rake/doc task

New jekyll needs 'build' as an argument. And the
configuration file has changed slightly.