This reverts commits deffc90, ab4dc64, and 0755579, returning to RJSONIO.
The purpose of this is to prepare for a maintenance release for 0.11
without the switch to jsonlite, to reduce the risk of new bugs.
These functions were temporarily ripped out of Shiny and moved
to the htmltools package. We've discovered that it's safe to
keep including them in shiny; as long as the functions in shiny
and the functions in htmltools are identical, the user won't
receive a conflict warning.