Ajax comments

Sorry about the glitch, my branch accidentally committed the last pull request too.
I added ajax properties to the comments, so you can now see new comments before refreshing the page.

Relevant files for this pull request:
app/controllers/comments_controller.rb
app/views/comments/_new.html.haml
app/views/comments/create.js.haml

I'm going to fix this git mess asap.

-Charles