DragAdorner upgrades

Added changes from https://code.google.com/p/gong-wpf-dragdrop/issues/detail?id=38, and added DragMouseAnchorPoint so the position of the DragAdorner relative to the pointer can be easily customized.  DragMouseAnchorPoint applies to explicitly specified DragAdornerTemplates, but not to images generated as default DragAdorners.