Python2 back compat

[vadimkantorov]

On my machine python3 segfaults due some nasty import issues, but python2 still works.

Here are a couple minor fixes to make the code run on python2 if one wishes so:

# in demo/experiment/util.py
import sys
if sys.version_info.major > 2:
    from urllib.request import urlretrieve
else:
    from urllib import urlretrieve

# in demo/experiment/voc.py
import sys
if sys.version_info.major > 2:
    from urllib.parse import urlparse
else:
    from urlparse import urlparse

[yeezhu]

Thank you!