Packing AFURLSessionManager to download files. Use ZWDownloadManager you create downloadtask, pause,continue,and continue transferring from breakpoint. 进一步封装AFURLSessionManager，实现下载任务的暂停，继续，断点续传，多任务下载等

###后续版本将会加入数据库存储下载进度等信息

###Before run the Demo,please runpod install ###请在使用前执行pod install