Render within internal app viewer when available

Note that this requires the master branch of shiny from github (for the ability to provide a function as the launch.browser parameter).

The RStudio internal application viewer is enabled in RStudio >= 0.98.414 (if an earlier version of RStudio is in use then it will gracefully fall back to using utils::browseURL). Note that for Shiny applications to work correctly inside RStudio the master branch of httpuv should be installed from github.
update readme with requirement of latest github versions for httpuv and shiny
Fix typo
Merge branch 'internal-viewer'