Support promisify(obj, string) promisify shorthand.

Occasionally, I need to `promisify` functions that are within deep objects or otherwise have long names, and need to maintain their context. Sometimes it is nice to avoid `promisifyAll` for performance reasons.

This PR allows the shorthand:

```javascript
var fn = Promise.promisify(a.deep.nested.object, 'aFunction');
```

Instead of the current

```javascript
var fn = Promise.promisify(a.deep.nested.object.aFunction, a.deep.nested.object);
```

I have added some basic documentation of the change in `API.md` but I am unsure how to document the parameters.