title: Swift extension for Dictionary. Error 'Cannot invoke 'dataWithJSONObject'
body: <p>I am newbie in swift. I have stuck in creating extension for Dictionary. 
  My extension is:</p>
  
  <pre><code>extension Dictionary {
      var JSONString: String {
          var error: NSError?
          var jsonData: NSData? = NSJSONSerialization.dataWithJSONObject(self, options: NSJSONWritingOptions.PrettyPrinted, error: &amp;error)
          if error == nil {
              return NSString(data: jsonData!, encoding: NSUTF8StringEncoding) as! String
          }
          return ""
      }        
  }
  </code></pre>
  
  <p>I get an error message </p>
  
  <blockquote>
    <p>Cannot invoke ‘dataWithJSONObject’ with an argument of type ‘(Dictionary , options:NSJSONWritingOptions, error: inout NSError?)’</p>
  </blockquote>
  
  <p>Can anybody help me to resolve this problem? </p>