Improve line removed markers / removed markers are confusing

Based on @felixkiss' suggestion from #20. This is tweaked a bit to make it look
reasonable when the gutter is wide. I also use the same technique when icons
are enabled. In the following images, line 14 has been removed.

Without icons:
![image](https://cloud.githubusercontent.com/assets/4427/3568036/822fc77c-0b28-11e4-8d8f-a8e495a66966.png)

With icons:
![image](https://cloud.githubusercontent.com/assets/4427/3568056/e565bebe-0b28-11e4-9df0-76e6dc49a72b.png)