Image __toString not correctly implemented

Image extends BasicField, and provides some abstract base methods - `__toString` causes an issue with Image in particular as it invokes `->get()` as get() for `Image` returns an Image instance.

`__toString` must always return a string.

Happy to shoot through a PR. Simplest solution would probably just to overwrite the abstract method __toString in image with something like:

```
/**
* @return string
*/
public function __toString()
{
return $this->url;
}
```

This was covered in #60 but it was closed because it wasn't explained in detail