Change API, the first argument is error, the second one is data

## After
```javascript
amesh.boundaryMapURL(function(err, data){
  data;// => http://tokyo-ame.jwa.or.jp/map/msk000.png
});
```
## Before
```javascript
amesh.boundaryMapURL(function(data){
  data;// => http://tokyo-ame.jwa.or.jp/map/msk000.png
});
```