code: public static <R> Observable<R> startCallable(Callable<? extends R> func) {
          return startCallable(func, Schedulers.computation());
      }
docs: Invokes the specified function asynchronously and returns an Observable that emits the result.
  <p>
  Note: The function is called immediately and once, not whenever an observer subscribes to the resulting
  Observable. Multiple subscriptions to this Observable observe the same return value.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/start.png" alt="">
  
  @param <R> the result value type
  @param func function to run asynchronously
  @return an Observable that emits the function's result value, or notifies observers of an exception
  @see <a href="https://github.com/ReactiveX/RxJava/wiki/Async-Operators#wiki-start">RxJava Wiki: start()</a>
  @see <a href="http://msdn.microsoft.com/en-us/library/hh229036.aspx">MSDN: Observable.Start</a>