Merge pull request #209 from ckeditor/i/203

Fix: Marked the editor instance `this.instance` as a raw object so that it will never be converted to a proxy. Closes #203.

Internal: Renamed the internal properties and methods: `$_instance` -> `instance`, `$_lastEditorData` -> `lastEditorData` and `$_setUpEditorEvents()` -> `setUpEditorEvents()`.

Author: pomek