Reorganizing Repo for Package Control

Can someone do a quick sanity-check on this change?

In order to be supported by Package Control plugin for Sublime, our repo needed to be reorganized to have all the plugin files in the root of the repo. Because this makes it harder for TextMate users now, I've created a second repo for just TextMate: https://github.com/dart-lang/dart-textmate-bundle

This change mostly just moves files around, but I also updated the README and fixed an issue with the Comments.tmPreferences file because it wasn't working.