change KnockoutComputed to extend KnockoutObservableAny instead of redefining

This enables proper polymorphism in computes toward observables.
upgraded JQueryDeffered.promise() to JQueryPromise.promise()

I didn't find this noted at jquery.com, however a quick runtime
inspection of "$.Deferred().promise()" in v1.9.1 will show that it is in
fact there.