FGGDownloader的swift版本一个基于NSURLConnection，用于断点下载及支持后台下载的框架 #Required 要求
iOS Version>=8.0
Xcode Version>=8.0
Swift Version>=3.0
#Install 安装:
Cocopods:
use_frameworks!
pod 'XGFDownloader'
Manual:
download XGFDownloader and drag it into your project。
#Useage 使用:
1.SET progress in the function create UI with: XGFDownloadManager.sharedManager.lastProgressWithUrl(url)
搭建UI时，设置显示进度的UIProgressView的进值:XGFDownloadManager.sharedManager.lastProgressWithUrl(url)
lastProgressWithUrl(url)returns a float value between 0.0 an 1.00.lastProgressWithUrl(url)方法的返回一个在0.0到1.0之间的Float类型的值；
2.Set file size/expect file total size label with text from: XGFDownloadManager.sharedManager.fileSize(url)设置显示文件大小/文件总大小的Label的文字：XGFDownloadManager.sharedManager.fileSize(url)
3.Resume or start downloading with. 开始或恢复下载任务的方法:
download(urlString:String, toPath: String, process:ProcessHandle, completion:CompletionHandle, failure:FailureHandle)
This function includes 3 call back blocks as follow: 1)during downloading call back block with 3 params: download progress->progress, downloaded part size->sizeString and downloading speed->speedString. speedString； 2)download finished call back block with no params. 3)downloading failed with error call back block with a param: error->error. 这个方法包含三个回调代码块，分别是： 1)下载过程中的回调代码块，带3个参数：下载进度参数progress，已下载文件大小sizeString，文件下载速度 2)下载成功回调的代码块，没有参数； 3)下载失败的回调代码块，带一个下载错误参数error。
#5.Explain 说明:
XGFDownloaderManager.sharedManager.cancelDownloadTaskWithUrlString(url:String)in the pause downloading function or in downloading failed call back block to cancel download task.
在下载出错的回调代码块中处理出错信息。在出错的回调代码块中或者暂停下载任务时，调用
XGFDownloaderManager.sharedManager.cancelDownloadTaskWithUrlString(url:String)方法取消/暂停下载任务；