Add #set_subreddit_sticky method

> Apologies to anyone that had started using the API to set posts as stickies, the method of setting it through settings will no longer work. There is a new API endpoint at /api/set_subreddit_sticky to set it now.

http://www.reddit.com/r/modnews/comments/1jwozg/moderators_there_is_now_a_button_on_the_comments/