colorize modified buffers in tabbar

Hi, 

I have tried your plug-in tabbar-ruler for Emacs recently and this perhaps the fanciest tabbar that I've used.

However, I saw that the modified buffers are not really shown perfectly. Hence, I made a minor changes that tabbar to display modified buffers name in orange colors.

If you find that my modification is helpful, please feel free to merge back into your original repository.

Best regards,
Quang Trung.