Bugfix: remove comparison shortcut

When comparing a non existing variable with
a value, != should return true instead of false.

Example $notexist != 'foo' should return true,
even if this is not supported in current XPath.
New attribute 'version' at cmd Layout

With the new attribute 'version' we can assure
that the layout file is processed with the
given version of the speedata publisher (or
higher). If version="1.5.44", the speedata
publisher will exit with an error if it's version
is less than 1.5.44 or if the major (1) or
minor (5) numbers differ.
Minor documentation corrections/update.