Swift TimeInterval 转换为 Int，秒数&毫秒数

参考：http://stackoverflow.com/questions/11121459/how-to-convert-nstimeinterval-to-int

1. 简单粗暴法：

let interval: TimeInterval = NSDate.timeIntervalSinceReferenceDate;
print("interval: \(interval)")
let time = Int(interval)
print("time: \(time)")
//结果为：interval: 496138595.969168
//time: 496138595

这样是直接截断，不太友好。

2. 四舍五入法：
   可精确到秒或毫秒：

  let aInterval = 496138080.543964
    //精确到秒
    let second = Int(round(aInterval))
    //精确到毫秒
    let millisecond = CLongLong(round(aInterval*1000))

    print("second: \(second), millisecond: \(millisecond)")
//结果为：second: 496138081, millisecond: 496138080544