remove special handling of speaker notes
clarify `reveal-prefix` alias name
update nbconvert docs about reveal.js
add proxying tornado server in Serve post-processor
use local reveal.js url prefix by default

CDN redirect is handled by the Serve post-processor if necessary
protect against unavailable tornado in post_processors.init
update reveal docs with recent changes
add nbconvert serve exclusions without tornado
install tornado in travis
Merge pull request #4048 from minrk/finish-notes

finish up speaker-notes PR

This removes the special handling of speaker notes, simply informing the user that speaker notes require local require.js.

It also renames the alias name for reveal.js url prefix to something more descriptive (reveal-prefix).