'Constructor of Microsoft COCO helper class for reading and visualizing annotations.
:param annotation_file (str): location of annotation file
:param image_folder (str): location to the folder that hosts images.
:return:'

def __init__(self, annotation_file=None):

    self.dataset = {}
    self.anns = []
    self.imgToAnns = {}
    self.catToImgs = {}
    self.imgs = {}
    self.cats = {}
    if (not (annotation_file == None)):
        print 'loading    annotations    into    memory...'
        tic = time.time()
        dataset = json.load(open(annotation_file, 'r'))
        print ('Done    (t=%0.2fs)' % (time.time() - tic))
        self.dataset = dataset
        self.createIndex()
