no issue
* move "save on first change" behaviour into editor controller
* allow TAB events to be specified in keyEvents hash of gh-input
* replace mobiledoc-kit/gh-koenig with a SimpleMDE based editor
- remove `gh-koenig` in-repo-addon from `package.json` so that test files etc aren't loaded
- remove `mobiledoc-kit` dependencies
- extends `gh-editor` to handle file drag/drop
- adds `gh-uploader` and `gh-progress-bar` components to handle file uploads in a more composable manner
- adds `gh-simplemde` component that wraps SimpleMDE
