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 think this looks like a good use of a view helper method.  Something like:

```ruby
module VotesHelper
  def vote_class_for(votable)
    vote = logged_in? && current_user.vote_for(votable)

    if vote.present?
      vote.upvote? ? 'upvoted' : 'downvoted'
    end
  end
end
```

This would require a change on a few things (i.e. remove the `user_vote` method on `Votable`, add a `vote_for` method on `User`, etc), but I think it's a bit cleaner than setting a local variable in a view.  What do you think?