Subscribe to deferred computed

Hi,

If you create a computed with "deferEvaluation: true", and then you never read the computed, but you subscribe to it, your subscription will never trigger, even if you update the underlying observables.  This pull request resolves that:

Example:
```javascript
var d = ko.observable(1),
    c = ko.computed({ read: d, deferEvaluation: true });

c.subscribe(function (value) { alert ("received " + value); });
d(10); // should trigger the subscription but it does not.
c();
d(11); // will now trigger the subscription
```

This is a trivial example, but imagine large applications where one piece of code creates the deferEvaluation computed (because the computation is expensive and not always used), and some other part of the application receives a computed that it should subscribe to.  It doesn't know if the computed is deferred or not.