Add Object handler to handle objects without specifing direct class

For now it is just for preview. Some implementation details need to be discussed.

### Problem:
Some times you have property which stores an objects of diferrent classes (subclasses of base class, interface implementations etc). Deserialization requires to know what real class object has.

### Solution:
Store an object class (type) in seriazlied format.

### Implementation:
Object handler and an **"object"** type which ads a type attribute to serialized object.
```php
/**
 * @Type(object)
 */
private $object
```
Attribute name can be changed
```php
/**
 * @Type(object<'attrName'>)
 */
private $object
```
_Docblocks, etc updates are required_

### Discussion:
* Is it conceptually right to add such feature in the core?
* An additional attribute is added to the object. Wll it be better to replace object with an array like
`$visitor->visit(array('type' => $type, 'data' => $object))`
* What ClassTypeNamingStrategyInterface default values are better?
* Only json and xml are implemented. Because i didn't find a good way to change YmlVisitor result (array with type and data may be a solution)
* Names of type, classes, etc.