Fix for image ordering issue

Fix for bug found here: https://github.com/rilwis/meta-box/issues/138
Since the menu
Fixed plupload ordering

Fixed the problem with the order of files.  Any images saved will now
have their `menu_order` auto incremented, which will caused them to be
added to the end of list of images.  Should finally solve
https://github.com/rilwis/meta-box/issues/138 and
https://github.com/rilwis/meta-box/issues/181