Start RxJava integration

Currently, creating an RxJava Observer from a Retrofit implementation is hard coding something like this:

```java

interface GitHub {

  @GET("/repos/{owner}/{repo}/contributors")
  List<Contributor> contributors(@Path("owner") String owner, @Path("repo") String repo);
}

// write an interface that invokes contributors on subscribe
class Contributors implements OnSubscribeFunc<List<Contributor>> {
  private final GitHub api;
  private final String owner;
  private final String repo;

  public Contributors(GitHub api, String owner, String repo) {
    this.api = api;
    this.owner = owner;
    this.repo = repo;
  }

  @Override public Subscription onSubscribe(Observer<? super List<Contributor>> observer) {
    observer.onNext(api.contributors(owner, repo));
    observer.onCompleted();
    return Subscriptions.empty();
  }
}

--snip--
// create a retrofit adapter normally
api = restAdapter.create(GitHub.class);

// create an observable which invokes the http call on subscribe
Observable<List<Contributor>> retrofitContributors = Observable.create(new Contributors(api, "square", "retrofit"));
```

This works, but it involves a lot of boilerplate coding to create the OnSubscribe function.  An alternate would be to allow Retrofit interfaces to extend RxJava function and coerce the function and its params to an `OnSubscribeFunc`.

Ex.
```java

// implement a function that can be coerced to an RxJava OnSubscribeFunc
interface Contributors extends Func2<String, String, List<Contributor>> {

  @Override @GET("/repos/{owner}/{repo}/contributors")
  List<Contributor> call(@Path("owner") String owner, @Path("repo") String repo);
}

--snip--
// create a retrofit adapter normally
api = restAdapter.create(Contributors.class);

// create an observable which invokes the http call on subscribe
Observable<List<Contributor>> retrofitContributors = Observable.create(toOnSubscribeFunc(api, "square", "retrofit"));
``` 

### Changes
  * The first commit in this pull request allows retrofit interfaces to extend functional (single-method) interfaces.
  * The second is optional, for now, as it has no retrofit code in it :)  It just shows how to do the above via.

cc @benjchristensen