Don't clear timers when runApp returns

The lifetimes of reactive expressions and observers can span multiple
runApp calls, so timers should as well.
bump the required version of httpuv to v1.2

this is to pull the changes to httpuv that enable shiny applications to run inside the rstudio viewer pane
Merge pull request #282 from rstudio/feature/httpuv-version

bump the required version of httpuv to v1.2
Merge remote-tracking branch 'origin/pr/276'

Conflicts:
	R/shiny.R
should pass a session object to invalidateLater()
add .csv to fileInput() to make it consistent with the tutorial (#280)
white spaces