Add voting

have fun merging this one helen!
So I just found this really cool way to do this with ActiveRecord.  You can apparently use the `sum` method to sum all of the values in a particular column: http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-sum.  So you can do something like this:

```ruby
votes.sum(:direction)
```

and you'll get the sum of the values in the direction column for that review's votes.  Pretty cool.  No need to change this, but you can if you want.