Allow for an optional drag handle to be specified

When defining the gridster, you can now define a draggable.handle selector:

For example:

    $("#widgets").gridster({
        widget_margins: [10, 10],
        widget_base_dimensions: [565, 400],
		draggable: {
			stop: function(event, ui) {
				console.log('save new columns');
			},
			handle: '.title'
		}
    });

If handle is omitted (or left blank), it works as it did previously. If a value is specified, then dragging only starts if the element which was clicked matches the selector specified.

Added by @vieron:
Closes #29

src/jquery.draggable.js

@@ -14,7 +14,8 @@
         limit: true,
         offset_left: 0,
         autoscroll: true,
-        ignore_dragging: ['INPUT', 'TEXTAREA', 'SELECT', 'BUTTON']
+        ignore_dragging: ['INPUT', 'TEXTAREA', 'SELECT', 'BUTTON'],
+        handle: null
         // ,drag: function(e){},
         // start : function(e, ui){},
         // stop : function(e){}
@@ -329,6 +330,10 @@
     };
 
     fn.ignore_drag = function(event) {
+        if (this.options.handle) {
+            return !$(event.target).is(this.options.handle);
+        }
+
         return $.inArray(event.target.nodeName, this.options.ignore_dragging) >= 0;
     };