[doctrine][model] allow store custom values.

This PR allow store any field even those that does not have mappings.

Since 0.7 this is possible:

```php
<?php

$details = new PaymentDetails;
$details['foo'] = 'theFoo';

$em->persist($details);
$em->flush();
```

the `theFoo`  will be stored to `others` column in the database.

The real life usecase: before 0.7 to store digital goods fields (L_PAYMENTREQUEST_0_NAME0 for example) you have to provide mapping in your class:

```php
<?php
/**
 * @ORM\Entity
 */
class PaypalExpressPaymentDetails extends PaymentDetails
{
    /**
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    protected $id;

    /**
     * @ORM\Column(type="json_array")
     */
    protected $l_paymentrequest_nnn_qtymmm = array();

    /**
     * @ORM\Column(type="json_array")
     */
    protected $l_paymentrequest_nnn_namemmm = array();

    /**
     * @ORM\Column(type="json_array")
     */
    protected $l_paymentrequest_nnn_descmmm = array();

    /**
     * @ORM\Column(type="json_array")
     */
    protected $l_paymentrequest_nnn_amtmmm = array();

    /**
     * @ORM\Column(type="json_array")
     */
    protected $l_paymentrequest_nnn_itemcategorymmm = array();
}
```

now this fields will be stored to `others` field by default and you still can define mapping