Fix bug where build failed if the diffs weren't built.
Merge pull request #113 from ndw/fix-xproc-build

Fix bug where build failed if the diffs weren't built.
Upgrade DocBook stylesheets to official 2.0.7 release
Make the cross-spec references more subtle.
Merge pull request #114 from ndw/update-docbook

Upgrade DocBook stylesheets to official 2.0.7 release
Merge pull request #115 from ndw/fix-xref-formatting

Make the cross-spec references more subtle.
Merge branch 'xproc20' of https://github.com/xproc/specification into xproc20
updating from xproc/specification