Makes toolbar buttons conform to the “plain” button style

+ Changes the background gradient of toolbar buttons.
+ Lightens the border around plain buttons
+ Adds plain buttons to the style guide

Before:
![screen shot 2014-11-12 at 10 06 59 am](https://cloud.githubusercontent.com/assets/186715/5015619/26b3a824-6a54-11e4-9809-7e370d10c71d.png)

After:
![screen shot 2014-11-12 at 10 06 21 am](https://cloud.githubusercontent.com/assets/186715/5015621/2addf6c0-6a54-11e4-8b9c-9452bf1c5d4c.png)

Styleguide:
![screen shot 2014-11-12 at 10 10 03 am](https://cloud.githubusercontent.com/assets/186715/5015617/1c820e72-6a54-11e4-8e59-6d4ba85344e3.png)