Voting

Allow for AJAXy voting on comments and articles. Resolves #14.


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I could throw something like `comment_count` or `num_comments` on articles. Whenever I add a new comment, I could do some form of recursion to find the article that comment is tied to, then increment said article's counter. That would make finding the number of comments on an article much easier and allow me to get rid of the `num_replies` method on `Comments`. However, for comments that are deeply nested, it might be resource intensive to go that route, as incrementing the counters would have a lot of recursive calls.