cordova文件傳輸系統插件使用：cordova-plugin-file-transfer

1. 添加插件：cordova plugin add cordova-plugin-file-transfer   

2. 調用方法：

var fileTransfer = new FileTransfer();
//下載地址;
var source="http://xxx/xxxx/ionic.apk";
var target="/sdcard/Download/ionic.apk";
var trustAllHosts = true;
var options = {};
function successCallback(entry){} ;
function errorCallback(error){};
fileTransfer.download(source,target,successCallback,errorCallback,trustAllHosts,options);
fileTransfer.onprogress = function(progressEvent){
    if (progressEvent.lengthComputable){
        $ionicLoading.show({
           template: "已經下載：" +Math.floor((progressEvent.loaded / progressEvent.total)*100) + "%"
        });
    }                                                                    
 }