fix some scrollbar issues

In my opinion the scroll bars in atom look really weird, because they don't have any margin.

## Changes I made
* added some pseudo margin using background-clip and a transparent border.
* have set the cursor to default on '.editor .vertical-scrollbar' because I got the text select cursor otherwise while grabbing the scrollbar thumb.
* remove the background color from ::-webkit-scrollbar-track to remove the background for the scrollbar in the settings view, thus making it much more visible

## Screenshots

![screenshot from 2014-05-06 21 55 00](https://cloud.githubusercontent.com/assets/3409958/2895012/995735d8-d55a-11e3-8706-0063b695e25a.png)
![screenshot from 2014-05-06 21 55 22](https://cloud.githubusercontent.com/assets/3409958/2895014/9fff2c4c-d55a-11e3-919f-84881eb9ad85.png)


## Comparison
![c1](https://cloud.githubusercontent.com/assets/3409958/2895052/1c4b4830-d55b-11e3-860a-aef2fe8d0b9e.png)