folderAdd = function(name) {
    // Check the name ends with a /
    if (name.slice(-1) != "/") {
        name += "/"; // IE doesn't like substr(-1)
    }

    // Does this folder already exist?
    if (!this.files[name]) {
        fileAdd.call(this, name, null, {
            dir: true
        });
    }
    return this.files[name];
}
Add a (sub) folder in the current folder.
@private
@param {string} name the folder's name
@return {Object} the new folder.