Default "this" improved for ".when"

Make sure the default "this." becomes the object you called ".when" on - and not the window object

Now you can write:

``` javascript
person.when(["firstName", "lastName"], function (firstName, lastName) {
  this.fullName = firstName + " " + lastName;
});
```

instead of 

```
person.when(["firstName", "lastName"], function (firstName, lastName) {
  person.fullName = firstName + " " + lastName;
});
```

Leaving your code more generic.