title: Django ImageKit: add watermark and create thumbnail
body: <p>I need to fit user's uploaded image to 1000px and put a watermark. Also I need to create thumbnail (without watermark).</p>
  
  <pre><code>class Watermark(object):
      def process(self, img):
          draw = ImageDraw.Draw(img)
          draw.line((0, 0) + img.size, fill=128)
          draw.line((0, img.size[1], img.size[0], 0), fill=128)
          return img
  
  
  class Photo(models.Model):
      image = ProcessedImageField(upload_to='photo',
                                  processors=[
                                      ResizeToFit(1000, 1000, upscale=False),
                                      Watermark(),
                                  ],
                                  format='JPEG')
      thumbnail = ImageSpecField(source='image',
                                 processors=[
                                     ResizeToFill(200, 200),
                                 ],
                                 format='JPEG')
  </code></pre>
  
  <p>The trouble is that the thumbnail is created from the already processed image.
  How to create a thumbnail from the original image, given that the original image should not be saved?</p>