Nicer tab items on Yosemite

I removed the text shadow and changed the font to regular weight, to match the style used throughout the system.

Before:

![screen shot 2014-10-20 at 16 11 37](https://cloud.githubusercontent.com/assets/183747/4702453/2b5be344-5863-11e4-9788-d1e9dec0c69a.png)

After:

![screen shot 2014-10-20 at 16 11 51](https://cloud.githubusercontent.com/assets/183747/4702456/2f3388f0-5863-11e4-9ff2-a77fccfa70ba.png)