Add isIn() helper function, use it to improve readability
Add isIn() helper function, use it to improve readability

* I think the need for this is justified by the existence of a few
  false or flaky conditionals due to misuses/typos of "typeof x ===
  y", etc.

* "typeof x === 'undefined'" is not needed anyway as we aren't
  targetting old browsers (or browsers at all for that matter) so we
  don't need to worry about compatibility, and it is noticably slower
  than "x === undefined"

GioBonvi/GoogleContactsEventsNotifier,baatochan/GoogleContactsEventsNotifier,rowanthorpe/GoogleContactsEventsNotifier