Twitter notifications

**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 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

**edit: this code has been reworked since I closed the original pull request.**
changes:
* Twitterable is still a concern, but its functionality is not 'mixed in' (ie it does not need to be included) to another model
* Twitterable's surface area has been decreased to a single method, 'tweet'. All other methods are private
* Event_instances' knowledge of Twitterable has been reduced
* Test coverage should be improved considerably.
* YAML settings to enable/disable the twitter feature are now working and tested
we should try to restrict testing one method for each `describe`/`context` block. This block tests two methods (`tweet` and `twitter_client`). It is easier to follow and isolate errors when done this way.