Make AnonymousSubject public

[metrofun]

AnonymousSubject seems to be a good starting point for creating our own Subjects by simple inheritance. For example:

function ThroughSubject(throughFunc) {
    var observer = new Rx.Subject();
    AnonymousSubject.call(this, observer, observer.map(throughFunc));
}
ThroughSubject.prototype = Object.create(AnonymousSubject.prototype, {
    constructor: {value: ThroughSubject}
});

Obviously the problem with the factory Rx.Subject.create is an inability to add anything to the prototype.

[mattpodwysocki]

@metrofun checked in and will be made available as Rx.AnonymousSubject