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 like the helper method. I implemented it a little differently though as I wasn't sure I liked having `vote_for` on the `User` model. Let me know what you think when I commit