Add Support for Promise#invoke method.

Hello!

This is a new method I find useful and that I think you might like.

Before:

```javascript
var p = new Promise();

fs.readFile('foo.txt', p);

var contents = p.get();
```

After:

```javascript
var p = new Promise();

var contents = p.invoke(fs.readFile, 'foo.txt');
```

(This defeats the purpose of asynchronous programming, to some extent, because it implicitly makes this action sequential, but I've found it quite useful in my own work.)