fix a bug that time duration may be 0 when downloading model binary

[gdh1995]

Sometimes urllib calls a hook the first time and then the second in a time distance too shorter than 1 second, in which case download_model_binary.py will break. This commit fixes this crash.

Extra information:

• When I ran scripts/download_model_binary.py to download caffe model, it once reported;

Traceback (most recent call last):
  File "***/caffe\scripts/download_model_binary.py", line 73, in <module>
    frontmatter['caffemodel_url'], model_filename, reporthook)
  File "d:\Python27\lib\urllib.py", line 93, in urlretrieve
    return _urlopener.retrieve(url, filename, reporthook, data)
  File "d:\Python27\lib\urllib.py", line 274, in retrieve
    reporthook(blocknum, bs, size)
ZeroDivisionError: float division by zero

• I saw the output file and found the count was about 6 KB.
• Windows 8.1 Pro x64, Python 2.7 (32 bits) with urllib-1.17

[shelhamer]

Thanks -- this doesn't seem like a common issue but it doesn't hurt to avoid it.