Update README.md
Fix reactivity for empty checkbox groups. Fixes #58
Update NEWS
Add reactivity tests
reactive tests: count number of times observers are run
Add test for reactive evaluation order
Add tests for isolate()
Use lazy evaluation of reactive functions
Add ctx$.label to help with debugging

Shows the code that the context "belongs" to.
Add execCount
Try LIFO pendingInvalidates?
Failing overreactivity test
Don't call observers until flushReact()
Remove all pendingInvalidate
Remove onInvalidateHint

The recent changes to onInvalidate make it do almost exactly the same thing.
Clarify isolation test
Un-export execCount
Add back onInvalidate to Observer class
Fix bad calls to on.exit

I didn't realize on.exit replaces previous calls to on.exit by default.
reactiveTable: don't return blank if first element is NA. Fixes #71