Load environment variables in dot-flatten form

last one was proposed to 4.2 branch here https://github.com/laravel/framework/pull/4617

Right now `.env` files allows only single dimensional array of config variables, on adding a multidimensional array like this

```php
return array(
    'database' => array(
        'mysql' => array(
            'host' => 'localhost',
            'user' => 'user',
            'password' => 'pass'
        ),
        'pgsql' => array(
            'host' => 'localhost',
            'user' => 'user',
            'password' => 'pass'
        )
    )
);
```
It throws error while trying to put keys and values in `$_ENV` and `$_SERVER` globals.
This patch converts array from `.env` file into a dot notation flatten array like this:

```php
array(
    'database.mysql.host' => 'localhost',
    'database.mysql.user' => 'user',
    'database.mysql.password' => 'pass',
    'database.pgsql.host' => 'localhost',
    'database.pgsql.user' => 'user',
    'database.pgsql.password' => 'pass',    
)
```

and lets you access them via dot notations. `getenv('database.mysql.host')` and so on