Flip (fixed) items-dropdown on top of trigger when there's not enough space below

This change flips the items dropdown on top of the select when there is not enough space to fold it out below. The change is only needed for cases where the dropdown has a `fixed` position. For cases where the dropdown position is `absolute` there will be a parent element that is either scrollable or has `overflow:visible`. (See here for a demo of the problem with the fixed dropdown: http://jsbin.com/UnOmIKI)

@grahamscott - Please review.


Thanks