Added Droppable

I needed a droppable target that would add items to an observable array without a sortable. Using the seating chart sample, it would allow you to drop students on table names with out having any template/child elements.

connectClass, beforeMove and afterMove, options, isEnabled all work the same (connectClass set's the droppable's accept)

An additional property, targetIndex, specifies where to put the new item, by default (null) it is appended. You can safely set to 0 to put at the start, or set it in beforeMove.

Every once in a while, I see a "Cannot call method 'removeChild' of null " on the source placeholder. Not sure what that is about, timing maybe. Appears to be this issue with JQuery UI: http://bugs.jqueryui.com/ticket/6054 

Brian
The following code looks at the "data" valueAccessor and if it is "observable" and it supports a splice (so, it fact this is an ObservableArray) then it insert or push the "item" into the observable.

Shouldn't something be done also for simple Observable, by setting its value to item?
Wouldn't targetParent better be named "target"?