WIP 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`

https://trello.com/c/nzVFiyco
How much do you care about this metadata? If the answer is "not much", I'd drop this class and have `Query#execute` return an array of Events.