Major refactor of runApp/addSubApp pipeline

- shinyUI and shinyServer calls are no longer required in ui.R and server.R
- shinyAppObj renamed to shinyApp
- runApp can take pathname, list(ui=..., server=...), shinyApp, and shinyAppDir
  as appDir argument
- Unify all Shiny app representations around shiny.appobj
- BREAKING CHANGE: shinyUI no longer has a "path" argument
- Instead of returning UI, ui.R can return a function that returns UI; it will
  be invoked each time the page is requested. (Note that this is NOT the same
  as saying ui.R will be run each time the page is requested.) The function can
  take either no args or a single "req" arg which is the request.