add element.blur()

[rodneyrehm]

Extracted from #81.

We need a utility to safely remove focus from an element.

• The utility should have an option to pass focus to the next focsuable ancestor (rather than always passing focus to body).
• We should guard from windows losing focus in IE9 as $.ui.safeBlur() does.
• The utility should accept an option that controls the behavior if the element to blur does not have focus.

Have a look at the focus fixup rules before implementing anything.

[rodneyrehm]

I've decided against featuritis. passing focus to another element should be done by focusing the other element explicitly. So this utility effectively becomes a guard against missing .blur() method on SVG and IE9's window-blur-problem.