Add #set_subreddit_sticky method

Uses the new API endpoint at /api/set_subreddit_sticky
Needed another parameter
API doesn't need a strict 'True' or 'False'

as paradox460 pointed out:

> We don't really need the implicit true/false there, as the API accepts anything starting with a T or Y as true and anything else as false. If we want to keep it, I'd prefer if we could switch to a double-bang notation, eg !!state, which accomplishes the same thing.
Publicize HTTParty wrappers