Added 'Collapse File Tree' feature

When working with a file tree, I miss the ability to collapse whole section recursively - so when I open the top directory again later, not everything under it will also be expanded. This is a feature for it.

Sample:
I want to collapse everything under "test" folder and when I open "test" folder again, I don't want to see "node", "node-modules", "fs-extra" and "lib" expanded too.
![image](https://f.cloud.github.com/assets/1067319/2288568/7c62c59e-9ffb-11e3-8943-72a96be19194.png)