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
@halogenandtoast Would you define that in the module w/ the other constants or in the class?