Yosemite tab close tweaks

Draw a round-rect background for tab close buttons to match Safari on OS X Yosemite. I tried getting the close icon to transition in and out on hover too, but it conflicts with the blue modified status indicator.

Hover over tab displays close icon:
![close](https://cloud.githubusercontent.com/assets/122102/5323073/45266bae-7c81-11e4-9db0-7a67bb2a9663.png)

Hover over close icon displays background:
![close-hover](https://cloud.githubusercontent.com/assets/122102/5323074/453df904-7c81-11e4-8ba7-86c1c7c47e92.png)