Minor revisions to element boilerplate

- Add more comments around when to use the various callbacks
- Remove `created` callback. We typically don't use this because it fires before polymer has had a chance to setup bindings. We favor `ready`.
- Add example `foo` published property and change watcher
- Version up polymer dependency
- Remove platform from bower.json (you'll get it automatically by asking for polymer)
Make src more vanilla
Merge pull request #13 from robdodson/update-callbacks

Minor revisions to element boilerplate