Do not strip whitespace from whitespace-only lines

When using the indent guides extension, the indents get interrupted if whitespace-only lines get stripped of their whitespace. It looks really disturbing and this fixes that issue. 

![screen shot 2014-10-30 at 19 41 24](https://cloud.githubusercontent.com/assets/7327050/4849808/6755041c-6064-11e4-9da1-0d22c725c4fc.png)