Allow widgets to be rendered into another frame

[philidem]

During widget initialization, it is currently assumed that the widgets will be found in the current window.document since document.getElementById(...) is used to find the widget elements that were rendered. I have a use case for rendering widgets into another frame. Something like:

var iframeEl = document.getElementById('some-iframe');
MyWidget.render({message: 'Hello'}).appendTo(iframeEl.contentWindow.document.body);

I have a fix that seems to be working properly and backwards compatible. I'll create pull request after a little more testing.