DPImageCache 介绍
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)
}
}