revealjs: speaker notes support in slim template + docs
revealjs: added more doc: data-background usage
Merge pull request #53 from obilodeau/revealjs/speaker-notes

revealjs: speaker notes support in slim template + docs
revealjs: expose all configuration through attributes (w/ doc)

Took the liberty to update available configuration parameters up to
reveal.js' current master (which is stable).
revealjs: document revealjs_theme and revealjs_customtheme attributes