#383 Parameter to terminate processing of version 4 documents.

[Simon Kennedy]

Add a parameter to terminate processing of a docbook version 4 document if processed with the docbook 5 style sheets.

Currently the docbook namespace is added to all nodes which can take some time.

Using <xsl:message terminate="yes"> a message could be output directing users to the correct formatting for a docbook 5 document could be added, before terminating processing.