Adding support for moment.add(1, 'second')

See #411.

This is to ease developer pain.

The following syntaxes are now supported.

```javascript
moment().add({seconds:1});
moment().add('seconds', 1);
moment().add('seconds', '1');
moment().add(1, 'seconds');
```

This syntax is not supported.
```javascript
moment().add('1', 'seconds');
```