Add support for asynchronous computed Observable

[pago]

If the value generated by an Observable.computed (on read) is a Promise, we should notify its listeners only after the Promise has been fulfilled. This will open various possibilities when used in combination with Observable#map, i.e.

var url = Observable('/foo.json');
var json = url.map(function(url) {
  return xhrPromise(url);
}).map(function(content) {
  return JSON.parse(content);
});

[pago]

Actually, that won't work and would require a huge amount of hacking. There's a better way to do it, though:

Add AsyncObservable, which is similar to a calculated observable expect that it notifies its listeners only after its value has been resolved. There should not be automatic dependency management for this type of observable (unlike ComputedObservable).

Usage:

var url = Observable('/foo.json');
url.async(function(urlObservable) {
  // we pass in the observable, not its value, to allow lazy evaluation
  // in case its a computed observable
  return xhrPromise(urlObservable.get());
}).map(function(content) {
  return JSON.parse(content);
});

Such an AsyncObservable starts with a value of null and is expected to return a Promise. Once that Promise is resolved, it will store the value and notify the listeners. If its dependency changes while a computation promise is running, it will ignore those changes. Maybe add an option to reevaluate the value once the running promise is resolved.

Primary use case: Make it much easier to delay the computation of an observable (i.e. to do so only on an animation frame).