Non-moderator setting of flairs

[lemtzas]

I actually already have some code that implements /api/flairselector and /api/selectflair but I'm unsure of how you would like it attached to the existing codebase. Currently it's in subreddit.rb alongside the Subreddit#get_flair and Subreddit#set_flair methods. Perhaps inside a thing/Flairable.rb mixin would be better?