FIX: Ensure GridFieldExportButton only has padding added when placed 'after'

Currently the `.grid-bottom-button` class is causing the button to be out of alignment with other buttons.

![screen shot 2013-06-20 at 12 13 56](https://f.cloud.github.com/assets/842280/686375/c656c0c6-da56-11e2-9da0-e8340e598fdc.png)

This is a simple fix to ensure that the class only gets added if the component is placed 'after' the GridField.