Twitter notifications

edit: we would appreciate a thorough code review on this feature
**Auto push pairing opportunities to twitter**
* Implemented general tweeting capability for Websiteone as a model concern (models/concerns/twitterable.rb) using the twitter gem (https://github.com/sferik/twitter).
* A tweet message is triggered when an event instance is saved with a hang_out url, or when an event instance is updated and the hangout_url has changed.
* In production mode, the twitter api keys are pulled from the environment
* In development and test modes, twitter api keys are set to nil unless overridden

@erpe, @coddeys  and I worked on this user story.
User story: https://www.pivotaltracker.com/story/show/65542324
`stub_const` could be an alternative to this approach, since it properly handles the removal of constants after the test