Return the reference to img obj in each Function.

With this, two or more operations can be squashed together in a single line.
For example, following 3 lines
```
   $image->scale(50);
   $image->resizeToHeight(500);
   $image->save('picture2.jpg');
```
can now be replaced with following single line.
```
   $image->scale(50)->resizeToHeight(500)->save('picture2.jpg');
```