Prevent tab focus from leaving dialog, fixes #3

Just closing out some of the modality support. I've tested on FF, Safari and IE10 (back to IE8 mode).
(There's a few TODOs, but the behaviour to fix them is too complicated for this first pull request).
I've made this.. more verbose, and just used `dialog.dialogPolyfillInfo` everywhere instead of `pfi`. I don't have a strong opinion either way, but I can see that the shorthand isn't that common - so that makes sense.