New expression language function: is_granted

Using the `is_granted` expression language function allows users to check permissions for the active user while describing the HATEOAS serialization properties, i.e.:

```php
/**
 * @Hateoas\Relation(
 *      "delete",
 *      href = @Hateoas\Route(
 *          "post_delete",
 *          parameters = { "id" = "expr(object.getId())" }
 *      ),
 *      exclusion = @Hateoas\Exclusion(
 *          excludeIf = "expr(not is_granted(['ROLE_ADMIN'])"
 *      )
 * )
 */
class Post
{
    // ...   
}
```

It is also possible to pass the object to the `is_granted` function:

```php
/**
 * @Hateoas\Relation(
 *      "delete",
 *      href = @Hateoas\Route(
 *          "post_delete",
 *          parameters = { "id" = "expr(object.getId())" }
 *      ),
 *      exclusion = @Hateoas\Exclusion(
 *          excludeIf = "expr(not is_granted(['ROLE_ADMIN', object])"
 *      )
 * )
 */
class Post
{
    // ...   
}
```

This pull request fixes issue #29 
Not really sure, I kind of copied the above function definitions...