Zorder

This works by having each subclass of D3Base explicitly set zorder in the _initialize() call based on the underlying matplotlib object. 

```
    def __init__(self, parent, ax, image, i=''):
        self._initialize(parent=parent, ax=ax, image=image,
                zorder=image.zorder)
```

I wanted to have the _initialize() method automatically pull it out of objects, but there didn't seem to be a way to know (in the cases like D3Image which pass more than one matplolib object) which was the correct matplotlib object to get zorder from.

I should go back in and add/modify a test case for this. Let me know if this is something you'd want to pull in and I'll do that.