issue,posting: Add Notification for comment.

* A user can watch by clicking watch button on issue or posting.
* The author, assignee and commenters become watcher automatically.
* Watchers receive a notification email when someone leaves a comment
  on the posting.
* Watchers can stop watching by toggling watch button.
Merge pull request #275 from npcode/feature/mail-notification-for-comments

issue,posting: Add Notification for comment.