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
Release version 0.2.5
Next development version
Allow ${version} to be Expanded in Other Files

Previously, only **/index.xml has placeholders expanded.

Add an optional property (expandPlaceholders) that will
permit a project to add a comma-delimited list of
additional file patterns that will be expanded.

Spring-AMQP currently has a maven snippet including the
version in quick-tour.xml.
Tweak CSS format of <p> tags in callouts

Include spacing between paragraphs in 'tip', 'note' and 'warning'
callout sections.
Add IDEA support to build.gradle
Allow project specific HTML overrides

Update DocbookReferencePlugin to allow users to include project
specific images and css files when generating HTML.
Allow project specific PDF images

Update DocbookReferencePlugin to allow users to include project
specific images when generating PDFs.
Merge pull request #10 from cwensel/project-overrides

# By Chris K Wensel
* project-overrides:
  Allow project specific PDF images
  Allow project specific HTML overrides
  Add IDEA support to build.gradle
Release version 0.2.6
Next development version