Operations Aggregate, Average and Sum with selector

Issue #653

Remarks:

- I know we can combine ops to get one of the new aggregate variant, but I think it might be worth having a direct version which avoids nesting several layers of Observables, Observers and Subscriptions.
- The ```averageInteger``` and ```sumInteger``` (and the other types) are handy if we want to use chained operation invocations (with less overhead):

```java
Observable.from("a", "bb", "ccc").sumInteger(s -> s.length())
    .toBlockingObservable().single();
```
instead of

```java
Observable.sumIntegers(Observable.from("a", "bb", "ccc").map(s -> s.length()))
    .toBlockingObservable().single();
```