check for descendants of drag-handle

(this is a updated version of my old pull-request at https://github.com/ducksboard/gridster.js/pull/326)
Function ignore_drag checks if event.target is a descendant of or the handle itself. This allows the handle to have child elements and dragging still works.
See http://jsbin.com/fuwunuyi/2 for a demo scenario: I added a span inside the header and now the widgets can't be dragged.