Extract video portion from Live Photo

Use the PHAssetResourceManager to get the video file from the PHAssetResource.

PHAssetResourceManager.defaultManager().writeDataForAssetResource(assetRes, 
    toFile: fileURL, options: nil, completionHandler: 
  {
     // Video file has been written to path specified via fileURL
  }

NOTE: The Live Photo specific APIs were introduced in iOS 9.1