Add multiple configures support

[jaceju]

Now, the attribute connection of model.php is used for set configure file.

Example:

<?php

use Mongor\Model;

class MyCollection extends Model
{
    public static $collection = 'my_collection';

    // Load options of "config/my_database.php"
    public $connection = 'other_database'; 
}

By default, connection is database for config/database.php.