Add subscriptions directly to a CompositeSubscription

[KenVanHoeylandt]

This feature is inspired by RxSwift DisposeBag.

The idea is that you can add your Subscription directly (inline) to a CompositeSubscription rather than having it assigned to a separate val first and then manually adding it a CompositeSubscription after assignment.

Usage:

Observable.just("test")
        .delay(100, TimeUnit.MILLISECONDS)
        .subscribe()
        .addTo(compositeSubscription)

I chose the name addTo() so that other containers could be targeted in the future.
addTo() returns the original Subscription so that it could be called consecutively.

A practical usage would be the Android Fragment instances:
They have several important life cycle events such as "UI destroyed" and "Fragment destroyed". Subscription instances can be tied to these life cycle events through CompositeSubscription instances that are unsubscribed when the life cycle event happens.

I've created an example and added tests for this feature.