Made editing more robust

I noticed that editing a comment failed on one of my projects. This was because I slightly changed the HTML structure and editing relies on that structure to stay exactly the same, see:
https://github.com/FriendsOfSymfony/FOSCommentBundle/blob/v2.0.4/Resources/assets/js/comments.js#L205

With this patch, editing becomes more robust because the "comment_body" is referenced by its id. On top of this, I made the container ID easily customizable by using a "data" attribute.