support working with elements in iframes

[rodneyrehm]

Any module accepting a context argument should use context.ownerDocument (instead of document) to query the DOM, in order to be able to work with elements in nested iframes. Currently this fails for (at least)

• utils/node-array (instanceof is not a good idea)
• get/insignificant-branches
• is/is.util
• query/focusable.strict
• when/focusable

That is about the scope we need to allow running the is/* tests against elements in an iframe.

To make this really work, we'd have to do quite some more:

• any use of the global references document and window need to go through element.ownerDocument and element.ownerDocument.defaultView respectively
• any use of the instanceof operator must be replaced by a window-independent alternative

We'll go for the simple solution (not covering window and instanceof) until someone actually needs to do this - in which case we'll expect another issue being opened.