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
after taking another look, I see what is happening. My initial thought was that `twitter_client` in this method is calling another method `twitter_client=` in the instance, however I now see it can also be interpreted as creating a new local variable `twitter_client` and assigning it to the new `Twitter::REST::Client` instance. However this may not behave as you expect it to, since the `twitter_client` variable does not live beyond the methods invocation, that means a new client is created every time you call this method. You may want to use an instance variable instead (`@twitter_client`), it also has the added benefit of being less confusing (at least to me!)