Add comments to Node.gitignore

I've rearranged the file patterns in the Node gitignore with comments to shed a bit more light on their purpose.

I also added a .gitignore for Cloud9, which is semi-related (C9 is all about Node).

Also, it'd be nice to see the "Node" .gitignore Github makes when you initialize your repository with a README updated with this content (right now I have to add "node_modules" every time).