Set outline styling when spark-button is focused.

I think the original webkit focusing style is ignored by bootjack(bootstrap). 
This makes `<spark-button>` possible to have focusing styling.

The CSS value is came from https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/css/html.css&q=html.css&sq=package:chromium&type=cs&l=1039

Review=@ussuri

![image](https://f.cloud.github.com/assets/756988/2004801/8ff91ff6-86c2-11e3-9abf-bed2346c842a.png)
![image](https://f.cloud.github.com/assets/756988/2004791/2881bc34-86c2-11e3-89b2-c7ca840bf13e.png)