Fixing a bug with export to PDF.

Chrome 33 caused (revealed?) a bug in the PDF export code. This was
fixed in reveal.js issue 808:

 https://github.com/hakimel/reveal.js/issues/808

This commit mirrors that fix for org-reveal.
Merge branch 'master' of github.com:krisajenkins/org-reveal
add title slide background
more robust and functional title slide attributes