EpicEditor.prototype.unload fails in an iframe

[tansongyang]

1. Create a page that uses EpicEditor. Make sure you call unload.
2. Load that page in an iframe.

You will get an error because of this line:

editor = window.parent.document.getElementById(self._instanceId);

This seems like a bug. Is there any reason to be calling window.parent here? Even in the first commit, this line was there, yet there were no other calls to window.parent. Everything works fine with just window.