Additional functionality for constraining where nodes may be dropped

Added option 'keepInContainer' to only allow nodes to be sorted within their original parent.

Added option 'maintainNestingLevel' to only allow nodes to be placed at the same nesting level as where they started.

Added option 'showErrorDiv' which decides whether a failed isAllowed() method shows an error placeholder or shows no change at all.

isAllowed() now takes the current item instead of the current placeholder.

I hope these changes may help :)