简单实现异步下载

异步：（需要实现delegate）
NSURLConnection 类：
+ (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id)delegate;
delegate 方法：
接收数据：
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response;
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;
http load完成：
- (void)connectionDidFinishLoading:(NSURLConnection *)connection;