Fixed KnockoutComputed polymorphism

Previously it was difficult to do polymorphic/covariant hotness such as...
```javascript
function getSomething(): KnockoutObservable<T> {
    if (something) {
        return ko.computed(...);
    } else {
        return ko.observable(...);
    }
}
```
...because KnockoutComputed was not typed as a valid KnockoutObservable.

I updated KnockoutComputed to inherit from KnockoutObservable, therefore enabling this situation.