Votes

Voting works for both articles and comments. Resolves #23. 

Rational for not-restful routes: no messy link / route changes asynchronously. Wanted to have a toggle so that clicking the vote again will destroy it (ala Reddit).
I'm not sure permitting a `user_id` here is wise.  I'd also argue that `article_id` and `comment_id` could also be removed.