Fixed the positioning and background

If the page is too long (as most website have html and body set to 100%), the modal overlay's gray background was covering only the current viewing area. If you were to scroll down, the user would be able to see the original stuff and click on with and thereby bypass the "barrier" that overlay provides. Also, the modal was positioned at the top. So, if you had scrolled down and then clicked on the modal initiating button/link, the modal could open up in the top, out of view of the user.