Functionality to allow multiple notifications

* This is a breaking change if the `formatter` option was being used
Notifications to allow work before and after deploy
Documentation update to ScreenNotifier
Airbrake notifier
NewRelic notifier
Merge pull request #1 from mattpolito/hashrocketeer-notifiers

New Notifiers
Merge pull request #17 from hashrocketeer/notifiers

Allowing 'callback' functionality through the use of notifications

* hashrocketeer/notifiers:
  NewRelic notifier
  Airbrake notifier
  Documentation update to ScreenNotifier
  Notifications to allow work before and after deploy
  Functionality to allow multiple notifications