DispatchKit 介绍
DispatchKit 能迅速简化中央调度。
创建和初始化队列：
let mainQueue: GCDQueue = .Main
let defaultQueue: GCDQueue = .Default
let customSerialQueue: GCDQueue = .createSerial("mySerialQueue")
let customConcurrentQueue: GCDQueue = .createConcurrent("myConcurrentQueue")