DPImageCacheUIImageView 扩展

DPImageCache 是 Swift 写成的 UIImageView 的扩展，帮助加载图像异步存储本地文件缓存。

使用：

let dirName = "cache"
func createCacheDirectory() {
        var fileMan = NSFileManager.defaultManager()
        var cacheDir = (NSSearchPathForDirectoriesInDomains(
            .DocumentDirectory,
            .UserDomainMask, true)[0] as! NSString)
            .stringByAppendingPathComponent(dirName)
        if !fileMan.fileExistsAtPath(cacheDir) {
            fileMan.createDirectoryAtPath(
                cacheDir,
                withIntermediateDirectories: false,
                attributes: nil,
                error: nil)
        }
    }