Implement NYT Events API wrapper

* Define `NytEvents` module
* Define `Event` and `Query` classes within module
* Update Gemfile with `dotenv-rails` and `httparty`
* Add `.env.sample` with sample NYT Events API key
Admins can import shows via the NYT Events API

* Add `nyt_event_id` and `nyt_updated_at` columns to `events`
* Add validation on `nyt_event_id`
* Create rake task for importing events from the API
* Create `EventsImporter` class
* Add helper method to `Event`
* Handle `NULL` `closing_date` values in `Event` and `date_helper`
* Remove unused methods from `NytEvents::Query`

https://trello.com/c/nzVFiyco