PHPLIB-186: Implement Manager accessors for core classes

[abellion]

Could be useful to get the manager from the Database class to instantiate the Collection class with the same manager. Example :

<?php

namespace App\Users;

use MongoDB;

class Users extends MongoDB\Collection
{

    //Get the Database class from the dependency injection engine (just for the example)
    public function __construct(MongoDB\Database $mongoDB)
    {
        parent::construct(
            $mongoDB->getManager(),
            $mongoDB->getDatabaseName(),
            "users"
        );
    }

    //Can now decorate the collection with specific methods :)

}

[jmikola]

Manually merged in b4ba7a7.

This was planned for some time in PHPLIB-186, but I had not gotten around to adding the methods. Thanks for the contribution!