Flush all shinyapp instances

Allows reactivity to affect all app instances at once.
(It already does but the outputs don't currently update)
Remove unneeded eval.parent
Observable: save visibility state of function
Add tests for reactive function return visibility
Merge pull request #89 from wch/reactive-invisible

Store visibility state of functions called from Observable
Merge pull request #85 from jcheng5/flush-all

Flush all shinyapp instances
Use correct default label for contexts. Fixes #91

NULL apparently is not a valid value for a field in a reference class.