Add computed option that allows the computed to "sleep" when it has no subscribers. Add ko.pureComputed to encapsulate that interface, emphasizing that this approach works best if the computed doesn't modify any external state. Update specs to use ko.computed instead of ko.dependentObservable.
Prevent recursive evaluation of sleeping computed.
Throw an error if a pure computed is evaluated recursively. Don't report on initial evaluation for pure computeds.
Don't expose isSleeping; auto-dispose pure computed if it has no dependencies; make copy of parameter object
Merge pull request #1359 from knockout/1359-pure-computed

Proposal: new computed variant that's "active" only when observed