Add voteScore, fix documentation, make read-only methods virtual

- Addresses issue #5 (vote scores)
- Adds documentation for the `.unvote()` method (which I did not know exists until I examined the source code)
- Switches the methods which just return a count to be [virtual getters](http://mongoosejs.com/docs/guide.html)
    - Provides a nicer syntax. Ex: `comment.upvotes` instead of `comment.upvotes()`