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