Use a CDN to get reveal.js library.
Make configurable the url where look for reveal.js library.
Fixing typo in reveal.tpl.
Added white line after imports.
Added some changes covering Mathias's commentaries.
Added look for key 'reveal' to not overwrite it.
Merge pull request #3533 from damianavila/reveal_cdn

Use a CDN to get reveal.js library

add reveal.url_prefix configurable