Fix button border when > 3 elem

Pull req for issue https://github.com/topcoat/topcoat/issues/460

This pull req is not yet ready. How it looks now

![image](https://f.cloud.github.com/assets/810040/1741805/2365f796-63eb-11e3-85c7-e1cedb79b462.png)

![image](https://f.cloud.github.com/assets/810040/1741806/31fcefda-63eb-11e3-97a2-7b98dfe11e82.png)

The way I did it.

````
.topcoat-button-bar__item:not(:last-child) > .topcoat-button-bar__button,
````

To disable the border on all elements except the last one. To my knowledge `:not` is a level 3 selector so as long as :last :first etc selectors work this should not be a problem.

The alternative would be to disable the border on all `.topcoat-button-bar__button` elements and set it again on the last one. Should not be a problem since the border style & size are defined via variables. 

Thoughts ?