Admins can import shows via the NYT Events API

**Implement NYT Events API wrapper:**
* Define `NytEventListings` module
* Define `Event`, `Query`, and `Result` classes within module
* Update Gemfile with `dotenv-rails` and `httparty`
* Add `.env.sample` with sample NYT Events API key
* Query the API on `events#index`

**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 to do the hard work
* Add helper method to `Event`

https://trello.com/c/nzVFiyco
I might *even* break this into a nicely-named helper method. Maybe `no_closing_date?(date)`?