Changed environment config loading from array_merge() to array_replace_recursive()

Small change when loading environment specific configurations. Now the code below will merge the username/password into database connections array instead of replacing it entirely.

~~~php
<?php
// my app/config/local/database.php
return array(
    'connections' => array(
        'mysql' => array(
            'username' => 'myname',
            'password' => 'mypass',
        ),
    ),
);
~~~