Add flatten() method

This is a method to make nested data to one-dimension array. Similar to Laravel `array_dot()`.

## Example

``` php
$array = array(
    'flower' => array(
        'sunflower' => 'light',
        'sakura' => 'samurai'
    )
);

$registry = new Registry($array);

// Make data to one dimension

$flatted = $registry->flatten();

print_r($flatted);
```

The result:

```
Array
(
    [flower.sunflower] => light
    [flower.sakura] => samurai
)
```