Hide 'revhistory' element

Hide the revision history element from default generated output. This
change allows docbook files generated from asciidoctor to render nicely.
Upgrade to Gradle 1.8
Upgrade JFrog build-info-extractor-gradle to 2.2.2
Move 'expandPlaceholders' to 'reference' ext

Move 'expandPlaceholders' into the 'reference' configuration rather
than relying on projects.ext.
Spaces -> Tabs
Add ability to configure fop-userconfig.xml.

With the ability to provide a user configuration of fop you can
add now new fonts to the pdf-renderer and build fancier output.
Merge pull request #19 from stephanheinze/master

* pull19:
  Add ability to configure fop-userconfig.xml.
Polish README