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
ok i'll move out the extra tests. your way might be marginally easier to debug, but i think this way reads better when running the tests, ie., none of these things should happen if the feature is disabled, and all of these things should happen if the feature is enabled.