Log when widgets are removed from sidebar areas

Fixes https://github.com/x-team/wp-stream/issues/33.

Works by checking for the class "deleting" on a widget being drug away from a sidebar. "Deleting" is added by WP WIdgets JS's droppable.  Then, if the "deleting" class exists, on drop (on mouseout), it sends the widget ID via ajax to the processor method to log the event.