Added centering of screen for multiple monitors

Proper pull for supporting multiple monitors. Note that while this fixes the issues for modern browsers, IE is only half fixed.

Old behavior:
No matter the browser, if centered, the pop up would be in the center of the first monitor

New behavior on standards browser:
If centered the popup will be on the center of the monitor which your browser is located.

New behavior on internet explorer browser:
If centered the popup will be on the center of the monitor which your browser is located. BUT, it will base the center coordinates off of your primary monitor. So if your monitor where your browser is located is at a different resolution than your primary, it may be slightly off center.