Using this function to save data string as image to library.
Ex:

```
var xhr = new XMLHttpRequest();
xhr.responseType="blob";
xhr.open("get", "http://www.mysite.com.tw/foo.jpg");
xhr.send();
var data = xhr.response.split(",")[1];
canvas2ImagePlugin.saveDataStringAsImageDataToLibrary(function(){console.log(true)}, function(){console.log(false)}, data);
```