Implement observability

[cjihrig]

For example, see vasync

[arb]

I like that idea.

[arb]

Were you thinking like each call would return like a standard observable object? Would it be something that inherits from EventEmitter or am I overthinking it?

[cjihrig]

I haven't thought about it a lot yet (been too focused on the tests). It could be hard to have a standard object that can be used with all the different methods. If we do go with an observable object, we should use a constructor function, as opposed to an object literal, to help v8 out.