Setting up proper gitignore for packages directory

Fair warning: I used `git rm --cached packages/npm` to remove **just the file path** from my local install. I'm not entirely sure what merging this PR will do to your installation. Will it remove just the path? Will it remove the sym-link? Will it remove the underlying npm directory in your ~/.meteor/packages directory, which is what happened when I tried a simple `git rm packages/npm` earlier?