title: DecisionTreeClassifier fit method error (scikit learn)
body: <p>Trying to train my <em>DecisionTreeClassifier</em> with <code>fit</code> method:</p>
  
  <pre><code>from sklearn import tree
  import skimage
  
  features = []
  labels = []
  
  for i in range(5):
      img = skimage.io.imread("circle" + str(i+1) + ".jpg")
      img = skimage.img_as_float(img)
      features.append(img)
      labels.append(0)
  
      img = skimage.io.imread("square" + str(i+1) + ".jpg")
      img = skimage.img_as_float(img)
      features.append(img)
      labels.append(1)
  
  clf = tree.DecisionTreeClassifier()
  clf = clf.fit(features, labels)
  </code></pre>
  
  <p>Receiving error:</p>
  
  <blockquote>
    <p><strong>ValueError:</strong> setting an array element with a sequence.</p>
  </blockquote>