Upvote

Resolves all 4 parts of Issue #3 
Resolves part 2 of Issue #10 
Resolves all 3 parts of Issue #11 
Move some of this logic to the review model.

```ruby
class Review < ActiveRecord::Base
  def upvote_count
    votes.where(vote: 'up').count
  end

  def downvote_count
    votes.where(vote: 'down').count
  end

  def cache_vote_count
    self.votes_count = upvote_count - downvote_count
    save
  end
end

class Vote < ActiveRecord::Base
  after_save do
    review.cache_vote_count
  end
end
```