Tunnel Shiny

This is not quite ready but I'm submitting it for review.

It fakes web sockets on the client and server side of Shiny, using RCloud ocaps to pass messages.

It uses the Inline Application embedded Shiny syntax:

http://rmarkdown.rstudio.com/authoring_embedded_shiny.html#inline-applications

.. substituting `rcloud.shinyApp` for `shinyApp`.

Known problems:
- there is a horrrrrible 3 second delay to load shiny.js, because we don't yet have an event for when knitted RCloud output DOM is ready for JS to annotate it.
- is it possible to have more than one Shiny app on a page?  Due to indiscriminate selectors, it looks like no, you will still have to use iframes if you want shiny in cells.