Create .numberFormatter()

Create format generator method called `.numberFormatter()`

```javascript
var formatter = Globalize( "en" ).numberFormatter();

[ 1, 2, 3.14159 ].map(function( x ) {
   return formatter( x );
});

// [ "1", "2", "3.142" ]
```

Ref #294
Fixes #295

- [x] Implement
- [x] Docs
- [x] Tests