Voting

Allow for AJAXy voting on comments and articles. Resolves #14.


![](http://zippy.gfycat.com/ImpracticalPastCardinal.gif)
Worth throwing this in an instance method on the `votable` class?  If not, this can be simplified to:

```ruby
votes = votable.votes

votes.where(is_up: true).count - votes.where(is_up: false).count
```

Is there a reason this needs to return a string?