Added GetThanksCount() method to Discussion, Comment and User model
Extended ThanksLogModel::Save()

* Method now returns the amount of thanks associated to the thanked object and to its author.
Completed implementation of "thanks" feature

* Implemented Ajax functions to send the "thanks".
* Added fields and methods to retrieve the amount of thanks received by Discussions, Comments and Users.
* Added extended DiscussionModel, CommentModel and UserModel.