Implemented toJSON function on Duration

Implemented toJSON function on Duration so JSON.stringify returns the ISO string representation of the duration

This is a fix for issue #1714 

```javascript
JSON.stringify(moment.duration(1024, 'd'));
```

Old Result: 
```javascript
'{"_milliseconds":0,"_days":1024,"_months":0,"_data":{"milliseconds":0,"seconds":0,"minutes":0,"hours":0,"days":4,"months":10,"years":2}}'
```

New Result: 
```javascript
'"P2Y10M4D"'
```