Minor fixes

VOKManagedObject class no longer exists.

For `countForClass:` and `arrayForClass:`: both mentioned using a temporary context, but neither takes a MOC argument. I think it would make sense for the method to check `[NSOperationQueue currentQueue]` and switch to a temporary context if necessary, but downstream, `safeContext:` is called anyway to get the context to use.

But, that leads me to the next thing: `safeContext:` doesn't actually return a safe context. If NOT on the main thread, shouldn't this get a temporary context from `temporaryContext` and return that?