text: Title: setup.py does not recognize opencv2 of Anaconda
  Body: When run the setup, error happens.  opencv is installed on Anaconda. 
  Is it possible to install imgaug on Anaconda?
  ...
  Processing ./dist/imgaug-0.2.0.tar.gz
      Complete output from command python setup.py egg_info:
      Traceback (most recent call last):
        File "<string>", line 1, in <module>
        File "/tmp/pip-K5MRPU-build/setup.py", line 6, in <module>
          raise Exception("Could not find package 'cv2' (OpenCV). It cannot be automatically installed, so you will have to manually install it.")
      Exception: Could not find package 'cv2' (OpenCV). It cannot be automatically installed, so you will have to manually install it.
  