Add more operator overloading to Observable and CompositeDisposable

[WendyYanto]

In this pull request, I'm proposing to add features in RxKotlin :

1. By using minusAssign operator, we can delete disposable from CompositeDisposable.

// Current
compositeDisposable.delete(disposable)
// Propose
compositeDisposable -= disposable

2. By using plus operator, we can merge Observable without calling Observable.merge explicitly. These features also support us to merge more than 4 observable because java Observable.merge() function overloads only to maximum of 4 observables.

// Current
val observable = Observable.merge(source1, source2, source3)
// Propose
val observable = source1 + source2 + source3

3. Add unit tests to previous plusAssign in CompositeDisposable

[isfaaghyth]

interesting!