Working download gist.

gistfile1.txt

    window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, function(fileSystem) {
            fileSystem.root.getFile("localFileName.txt", {create: true, exclusive: false}, function(fileEntry) {
                    var localPath = fileEntry.fullPath;
 
                    if (device.platform === "Android" && localPath.indexOf("file://") === 0) {
                        localPath = localPath.substring(7);
                    }
 
                    var ft = new FileTransfer();
 
                    ft.download("https://dl.dropboxusercontent.com/u/887989/guinness.jpg", localPath, function(entry) {
                            console.log("successful download");
                        }, failFn);
                }, failFn);
            }, failFn);