add folded line icon

Simple tweak now that the Gutter API landed!

Screenshot:
![image](https://cloud.githubusercontent.com/assets/253202/3225392/a4b2f92a-f04f-11e3-8ab3-51cfc999641a.png)


This line should come out. Checking `isFoldedAtBufferRow` for every render call is actually pretty expensive. We already have a decoration following around the fold markers. So just take the `type: 'gutter'` out of this line: https://github.com/atom/atom/blob/master/src/display-buffer.coffee#L1058 and it should work.