Make sure that a subreddit actually exists.

def check_exists(subreddit):
    """Make sure that a subreddit actually exists."""
    req = get('http://www.reddit.com/r/%s/about.json' % subreddit, headers={'User-Agent': 'CslBot/1.0'})
    if req.json().get('kind') == 'Listing':
        # no subreddit exists, search results page is shown
        return False
    return req.status_code == 200