fixed exception for posts with no tags

[bog4t]

For the python verison of the downloader, I recieved this exception:

line 135, in download
    for tag in s.find(class_='tags-row').findAll(class_='tags'):
AttributeError: 'NoneType' object has no attribute 'findAll'

I made a quick fix by wrapping the for loop with a try and except block.

[egypturnash]

This fixes #67, thank you, I now have untagged giant titties on my local drive. :)