Add solution for decorator problem

Having trouble implementing the exact interface described in the gist. I can use my solution as follows.

```
var addCallCount = countDecorator(Add);

addCallCount.count();
// -> 0

addCallCount(1,2);
// -> 3

addCallCount.count();
// -> 1
```

I think I am using the closure correctly, but I'm not sure how to get the [desired usage](https://gist.github.com/hankyates/9680619).