Prototyping Simplified Comment

* You can attach simplified comments without modifying entity classe.
* You can add new comment with "SimpleCommentApp.newComment(String resourceKey)".
	* The 'resourceKey' is unique value for the entity you want to attach comments.
	* ex) pull_request_120, milestone_12
* You can get the comments with "SimpleComment.findByResourceKey(String resourceKey)"
* This is working now for the PullReuqest's comments.
	* So you can checkout on the PullRequest View page.