Re-assess volatile

[janechu]

Description

Currently volatile relies on interpretation of a conditional to determine what properties will cause it to update.

Proposal

Consider the common use cases for volatile and determine if there is a more reasonable implementation that would be easier for developers to debug.

A crude example:

Observable.update(
    [this.a, this.b], // observable properties to attach a watcher to
    this.c // property that will have trigger Observable.notify() against
);