Clarified documentation for multiplex plugin.

Thanks to @davidbanham for his help.
correction to navigate top/right/bottom/left methods (closes #409)
Merge pull request #398 from jmdobry/multiplex-docs

Clarified documentation for multiplex plugin.
adjust order in readme
remove ?notes from readme
ie8 tweaks
Merge branch 'master' of https://github.com/hakimel/reveal.js into dev