Implement as()-based converter API

[ZacSweers]

From the beginning, the ideal case has been for AutoDispose to be able to have a single point of entry API. to() unfortunately made this hard since each API accepted the same Function type with different generics. After contributing custom interfaces support in ReactiveX/RxJava#5729 though, the road ahead is clear.

The plan would be for AutoDispose to remain in pre-stable until as() is promoted to stable API (possibly in RxJava 2.2).

The implementation would be straightfoward. There would be a static "entry point" method with overloads for different scopes. Under the hood they would each just return an OmniConverter that implements all the converter interfaces. This would be the "final" API of AutoDispose for 1.0

Open question - What should the "entry point" method be named?

In java 8, you won't need the generic, which means you could have APIs like this:

Observable.just(1)
    .as(autoDisposable(scope))
    .subscribe();

For Java 7, you need the generic, and subsequently can't static import the method:

Observable.just(1)
    .as(AutoDispose.<Integer>autoDisposable(scope))
    .subscribe();

Note: In IntelliJ 2017.3, the generic above will autocomplete

In the Java 7 case above, it feels a bit... redundant? Idk. It might also be nice to unify this name and the kotlin extension functions, which are currently all autodisposeWith