Feature/owner security voter

Permits to use the decision manager system to check if an object implementing `OwnableInterface` is owned by an object implementing `OwnerInterface`.

If object implementing `OwnerInterface` also implements `EquatableInterface` and `UserInterface`, it will be used to check equality of owner.

``` php

class Post implements OwnableInterface
{
    /**
     * @ORM\ManyToOne(targetEntity="User")
     **/
    public $createdBy;

    public function getOwner()
    {
        return $this->createdBy;
    }
}

class User implements OwnerInterface, UserInterface, EquatableInterface
{
}


$securityContext->isGranted('IS_OWNER', new Post); // true if Post::createdBy is current logged in user

```