Center parent

Proper pull for centering based on parent.

This allows users to center the pop up based on the browsers window, regardless of monitor. It uses the existing property but changes from boolean to string, while still maintaining backwards support.

Although the existing behavior is to center on `'screen'`, I think you should consider changing the default to center `'parent'`. Most users will not know the difference, and it is more compatible with multiple monitors (see ie bug from #8).