Fix use billing address checkbox

So currently the inline checkbox for "use billing address" is mis-aligned and not following bootstrap instructions (http://getbootstrap.com/css/#forms). This PR fixes it. Screenshots of before & after attached.

![screen shot 2014-05-30 at 9 55 11 am](https://cloud.githubusercontent.com/assets/852746/3131306/0f09d65e-e803-11e3-91be-8d8a22a699d3.png)

![screen shot 2014-05-30 at 10 00 04 am](https://cloud.githubusercontent.com/assets/852746/3131308/128b5fc8-e803-11e3-9886-9880dba2c334.png)