Conditionally append lean_overlay div to doc body

Without this change, leanModal can only be called once. On subsequent calls additional, spurious #lean_overlay divs are added. Multiple copies of the same id should not be created.
Changed "top" option to allow non-integer value

Useful when setting top option to value other than pixel. ie. "100%", "1em"
Added preventDefault to modal close button
add param to support jQuery 1.8x
Update jquery.leanModal.js
JSHint fixes, whitespace cleanup, and consistent space/tab usage
add esc key support