'Download COCO images from mscoco.org server.
:param tarDir (str): COCO results directory name
imgIds (list): images to be downloaded
:return:'

def download(self, tarDir=None, imgIds=[]):

    if (tarDir is None):
        print 'Please    specify    target    directory'
        return (-1)
    if (len(imgIds) == 0):
        imgs = self.imgs.values()
    else:
        imgs = self.loadImgs(imgIds)
    N = len(imgs)
    if (not os.path.exists(tarDir)):
        os.makedirs(tarDir)
    for (i, img) in enumerate(imgs):
        tic = time.time()
        fname = os.path.join(tarDir, img['file_name'])
        if (not os.path.exists(fname)):
            urlretrieve(img['coco_url'], fname)
        print 'downloaded    {}/{}    images    (t={:0.1f}s)'.format(i, N, (time.time() - tic))
