add stats property for Coco class

[fcakyon]

coco.stats will return dataset stats:

{
    'avg_annotation_area': 2448.405738278109,
    'avg_num_annotations_in_image': 53.037243084530985,
    'max_annotation_area': 328640,
    'max_num_annotations_in_image': 902,
    'min_annotation_area': 3,
    'min_num_annotations_in_image': 1,
    'num_annotations': 343204,
    'num_annotations_per_category': {
        'human': 106396,
        'vehicle': 236808
    },
    'num_categories': 2,
    'num_images': 6471,
    'num_images_per_category': {
        'human': 5684,
        'vehicle': 6323
    }
}