Fixing header comments and formatting

Mostly for packaging... If you look at http://melpa.milkbox.net/#/polymode you'll see that your whole file is in the description. I _think_ I fixed that, but you might want to look at http://www.emacswiki.org/emacs/ElispAreaConventions and http://www.emacswiki.org/emacs/header2.el to make sure.

I also fixed some minor typos and a broken url.