'Delete rejected articles, if there is an articles param,
purge from there, otherwise purge from source instance.
Reference this StackOverflow post for some of the wonky
syntax below:
http://stackoverflow.com/questions/1207406/remove-items-from-a-
list-while-iterating-in-python'

def purge_articles(self, reason, articles):

    if (reason == 'url'):
        articles[:] = [a for a in articles if a.is_valid_url()]
    elif (reason == 'body'):
        articles[:] = [a for a in articles if a.is_valid_body()]
    return articles
