Added docbook-xsl-saxon dependency

Added 'net.sf.docbook:docbook-xsl-saxon:1.0.0' as a dependency to the
docbook-reference-plugin. This dependency is required for handling
callouts in docbook (i.e. <programlistingco> etc.).
Merge pull request #8 from poutsma/callout_support

# By Arjen Poutsma
# Via Arjen Poutsma
* callout_support:
  Added docbook-xsl-saxon dependency