Update score circle icon to look great on HiDPI displays, as well as fitting better with WordPress 3.8+ design.

This pull request updates the score icon to use a CSS border-radius instead of an image. It now fits better with the WordPress 3.8+ design along with HiPDI displays. The colours that I've used are in core WP itself for more consistency. They are the colours used on the left border of notification alerts.

I've also updated the positioning inside the Publish metabox along with the admin bar to be consistent with the other items.

Comparison screenshots:

![score-list-view](https://cloud.githubusercontent.com/assets/704594/2744814/2fce8288-c72c-11e3-80da-115aab1908a2.png)

![score-publish-metabox](https://cloud.githubusercontent.com/assets/704594/2744817/3b234d80-c72c-11e3-929a-7ee5379558df.png)

![score-admin-bar](https://cloud.githubusercontent.com/assets/704594/2744859/d5bc277c-c72c-11e3-892d-13b03641b61a.png)