[MRG+1] adding conditional import to lfw.py for python3 support

I tried to run this command from python 3.4:
`lfw_people = datasets.fetch_lfw_people(min_faces_per_person=70, resize=0.4, data_home='datasets')`

I got the following error:

    /usr/local/lib/python3.4/site-packages/sklearn/datasets/lfw.py in check_fetch_lfw(data_home, funneled, download_if_missing)
         85                 url = BASE_URL + target_filename
         86                 logger.warn("Downloading LFW metadata: %s", url)
    ---> 87                 urllib.urlretrieve(url, target_filepath)
         88             else:
         89                 raise IOError("%s is missing" % target_filepath)

    AttributeError: 'module' object has no attribute 'urlretrieve'

Urllib in Python3 no longer supports urllib.urlretrieve.  It is now under urllib.request.