Hide the tab bar border when empty

Adds a simple selector to hide the tab-bar border when there are no tabs to be displayed. This removes the very unsightly thick line below the title bar, as you can see here:

![tab-bar-border-fix](https://f.cloud.github.com/assets/4340/2296679/cf361932-a0a1-11e3-9573-5097182e1ac8.gif)