Update on the API regarding closures involving sharedSessions & relative URLs in Swift needs to be noted.

override func viewDidLoad() {
    super.viewDidLoad()
    let baseURL = NSURL(string: "https://api.forecast.io/forecast/\(apiKey)/")
    println("Base URL", baseURL)
    let forecastURL = NSURL(string: "37.8267,-122.423/", relativeToURL: baseURL)

    //let weatherData = NSData(contentsOfURL: forecastURL!, options: nil, error: nil)
    //println(weatherData)

    let sharedSession = NSURLSession.sharedSession()

    let downloadTask: NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(forecastURL!, completionHandler: { (location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
      println(location, response)
    })

    downloadTask.resume()

  }