Copied docbook resources to build directory before processing

Copied the handbook and all XSLs to build directory before to avoid references
like ../../../target/xyz in the source files.
Split copying of handbook and examples XSLs
Built handbook examples in separate AntRun execution
Extracted building of handbook examples to separate build-examples.xml
Building examples in different dir than handbook content
Used Docbkx Maven Plugin to build handbook.html

Built unix-handbook:html:single with docbkx:generate-html and attached it with
build-helper:attach-artifact. Removed the corresponding targets from from
build.xml. Had to update the handbook-html-single.xsl customization stylesheet
to load the DocBook stylesheets from docbkx. Configured the parameters in
handbook-common.xsl directly as plugin parameters. Used XML Maven Plugin to
apply identity.xsl and generate-id.xsl.
Used Docbkx Maven Plugin to build chunked html handbook

Built chunked html handbook and assembled it with Maven Assembly Plugin.
Removed corresponding ant target.
Used Docbkx Maven Plugin to build pdf handbook
Reduced duplication in docbkx-m-p configs

Shared configuration between executions of Docbkx Maven Plugin. Also deleted
unused handbook-common.xsl after abandoning AntRun for manual DocBook XSL
transformation.
Deleted unused handbook dependencies and properties

Cleaned up after abandoning AntRun for handbook building: Deleted properties
version.docbook, version.maven-ant-tasks, version.unix-maven-plugin. Deleted
dependencies org.apache.maven:maven-ant-tasks, saxon:saxon,
org.apache.xmlgraphics:fop, xerces:xercesImpl,
net.sf.docbook:docbook-xsl:zip:ns-resources, net.sf.docbook:docbook-xsl-saxon,
net.sf.xslthl:xslthl.
Removed unused xslthl repository
Deleted clean target from build-examples.xml in handbook
Refrained from copying handbook examples to unused dir target/docbook
Handbook POM indentation
AntRun <tasks> -> <target>
Read handbook examples and XSLs directly from src/
Merge pull request #7 from knutsiem/docbkx-maven-plugin

Use Docbkx Maven Plugin to build handbook