Add an authorName property to comment model

I've added a `authorName` property which is added only when `Comment` entity does not implements `FOS\CommentBundle\Model\SignedCommentInterface` which is for use of `FOSUserBundle` and authenticated feature.

Here is the view:
![capture decran 2014-02-13 a 15 38 41](https://f.cloud.github.com/assets/103900/2160718/a07aa298-94bc-11e3-83d7-646998576670.png)