1.Hot & Cold Observables
（1）Cold Observables：在被订阅后开始产生数据，即这类observables序列仅在subscribe()被调用后才会推送数据；
let obs = Rx.Observable.create(observer => observer.next(Date.now()));
obs.subscribe(v => console.log('1st subscriber:' + v)); //1st subscriber: 1465990942935
obs.subscribe(v => console.log('2st subscriber:' + v)); //2nd subscriber: 1465990942936
//两次获得的数据不同，即observer.next(Date.now())被调用了两次；
（2）Hot Observables：在被订阅之前就已经开始生成数据；
