Add downvote (fixes #43)

This PR has the intention of adding downvoting to the list of options for a user if that user can downvote.  I was not able to fully test this because I don't have downvoting privileges.  However, when using the html provided by @wting in Issue #43 and forcing his username the option did appear.  Before merging this in it would probably be wise for someone who has downvoting privileges to pull my branch and run it to make sure everything is kosher.  

That being said, given that the display shows up properly and the fact that the voting logic is pretty trivial I'm confident that this works.