Improved open and closed folder icons

I think the closed folder icon is too harsh against the dark background and doesn't fit well with the other folder/file icons.
![seti-folders](https://cloud.githubusercontent.com/assets/814402/4877859/fbe13578-62f9-11e4-8395-f967c1b3990d.png)
I've updated the closed folder icon to fit better with the open folder icon with a 50% opacity and slightly smaller in size. The difference is far easier on the eyes and folder open/closed can still be differentiated at a glance.
![seti-folders-new](https://cloud.githubusercontent.com/assets/814402/4877879/6c56bd6e-62fa-11e4-8942-94b696d7fc5c.png)