操作符总结：http://reactivex.io/documentation/operators.html
Operators By Category
Creating Observables
Operators that originate new Observables.
Create— create an Observable from scratch by calling observer methods programmatically- — 基本创建方式
Defer— do not create the Observable until the observer subscribes, and create a fresh Observable for each observer- — 当观察者订阅时，才创建Observable，而不是像Create一样，执行Create就创建好了，在subscribe时才创建这个对象。并且针对每个观察者创建都是一个新的Observable。 RxJava的懒加载，慎重使用自定义操作符，优先考虑内置操作符
Empty/Never/Throw— create Observables that have very precise and limited behaviorFrom— convert some other object or data structure into an ObservableInterval— create an Observable that emits a sequence of integers spaced by a particular time intervalJust— convert an object or a set of objects into an Observable that emits that or those objectsRange— create an Observable that emits a range of sequential integersRepeat— create an Observable that emits a particular item or sequence of items repeatedlyStart— create an Observable that emits the return value of a functionTimer— create an Observable that emits a single item after a given delay