Localized number formatting

Let's add support to localized number representation.
For instance, with the locale it_IT the decimal separator is `,` and the thousands separator is `.`.
With this helper we have:
```php
echo Loader::helper('number')->format(123456.78);
```
results in `123.456,78`
```php
echo Loader::helper('number')->unformat('123.456,78');
```
results in `123456.78`