Adding ability to change popup icon dynamically

Hi  @gabrielemariotti 

I needed ability to change popup drawable (icon) dynamically so I made it for my app & thought to share with everyone hence created a pull request.

There is small catch to this technique -
As I have set default popup drawable to NULL in style.xml, we need to always specify drawable from Java code as shown below.

Add this line in where you have used CardHeader
```
header.setPopupButtonDrawable(R.drawable.your_drawable);
```