don't use knitr cache by default for presentations
show progress after clearing knitr cache
use blank image for empty plots pane
Merge branch 'master' of github.com:rstudio/rstudio
Merge remote-tracking branch 'origin/master' into feature/error-management

Conflicts:
	src/cpp/session/SessionClientEvent.cpp
	src/cpp/session/include/session/worker_safe/session/SessionClientEvent.hpp
	src/gwt/src/org/rstudio/studio/client/server/remote/ClientEvent.java
	src/gwt/src/org/rstudio/studio/client/server/remote/ClientEventDispatcher.java