Add functionality to Loggable annotation.

I need to log fields declared at vendor's bundle (FOSUserBundle/User), I managed to do with this fix.


Sample:
````
/**
 * BaseUser is at vendor. So we cannot modify it.
 */
class BaseUser {
	protected $username;
}
```
---
```
/**
 * We want log all changes on 'username' property.
 * @Gedmo/Loggable(propertyNames={"username"})
 */
class DerivedUser extends BaseUser {
	protected $id;
	/**
	 * @Gedmo/Versioned
	 */
	protected $email;
}
```
---

This extension allows all changes on $username or $email will be logged.