Make expand instantiate classes within the calling object

Currently, if we do 
```
product = builton.product().get(id="5d67c87ad661690e5f6250a6", expand="company")
```
we get 
```
type(product.company) == dict
```
We could instantiate a new object instead of treating it like a dict. For that, we could use the `_cls` key in the dict expanded to decide which model we should use. 