Storing keras-retinanet package

Recent changes to the keras-retinanet github (https://github.com/fizyr/keras-retinanet) posed the problem that 0.5.0/0.5.1 source code and pre-trained ResNet50 with CoCo labels won't be available in the future. As the keras-retinanet project is Apache License 2.0 at this moment (https://github.com/fizyr/keras-retinanet/blob/master/LICENSE), we store the source code and pre-trained used by chapter 13 in this repository for readers to download.