Voting

Allow for AJAXy voting on comments and articles. Resolves #14.


![](http://zippy.gfycat.com/ImpracticalPastCardinal.gif)
Hmm, I think that the reason I put these in here originally was for the purposes of routing. I was originally thinking that putting them in `ArticlesController` and `CommentsController` would make them easy to find, but, as I think about it more, the routes imply they should be associated with a `Votable` concern, which would make `Votable` the most logical (and DRY) place to put them.