title: cannot import xfeatures2d in python OpenCV binding
body: <p>I am trying to access SIFT and SURF features from opencv 3 in python. As suggested by other people, I followed the installation of opencv_contrib. Some of the logs of the installation are:</p>
  
  <pre><code>[ 82%] Built target opencv_xfeatures2d
  ...
  Installing: /usr/local/lib/libopencv_xfeatures2d.so
  -- Set runtime path of "/usr/local/lib/libopencv_xfeatures2d.so.3.0.0" to "/usr/local/lib"
  -- Installing: /usr/local/include/opencv2/xfeatures2d.hpp
  -- Installing: /usr/local/include/opencv2/xfeatures2d/cuda.hpp
  -- Installing: /usr/local/include/opencv2/xfeatures2d/nonfree.hpp
  </code></pre>
  
  <p>After that I import cv2 in python but could not import xfeatures2d.</p>
  
  <pre><code>&gt;&gt;&gt; import cv2
  &gt;&gt;&gt; cv2.xfeatures2d.SIFT_create()
  Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  AttributeError: 'module' object has no attribute 'xfeatures2d'
  </code></pre>
  
  <p>Any clue?</p>