Add ko.computedContext to replace/supplement ko.dependencyDetection and more

[mbest]

ko.computedContext can include some public functions such as

• getDependencyCount - return number of dependencies so far in current evaluation. see knockout/knockout#627
• isDependency - return whether an observable is a dependency in current evaluation
• isInitial - is this the first evaluation of the computed?
• didChange - if the given observable was previously a dependency, return whether this evaluation was due to a change in that observable (only really useful when the computed is used as a reactor) see #22
• computed - the current computed observable