add Observable.sort() instance methods

Added two ```Observable.sort``` instance methods that are equivalent to 
```java
Observable.toSortedList().flatMap(x -> Observable.from(x))
```
I guess you might ask well why don't people just code that. I think it adds the following:

* a more natural reading api method
* doesn't interrupt the observable flow with buffered type ```Observable<List<T>>```
* less mess for those people coding without lambdas