[refactored] ReactiveProperty to be an IndefiniteSubject subclass
[refactored] ReactiveProperty to be an IndefiniteSubject subclass

Summary:
Benefits:

- It passes `isObservable()`, and can thus be used as an argument to `_reactiveMap` and friends
- It exposes `next(value)` publicly, which makes it possible to do `otherStream.subscribe(property)`

Reviewers: O2 Material Motion, O3 Material JavaScript platform reviewers, #material_motion, featherless

Reviewed By: O2 Material Motion, #material_motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D3226

material-motion/material-motion-js,material-motion/material-motion-js,material-motion/material-motion-js