DnD 'start distance' feature

This feature can be used to prevent unwanted drags when clicking on an element.

Set global value:

```javascript
Kinetic.DD.distance = 5;
```

Set `dragDistance` attribute:

```javascript
var rect = new Kinetic.Rect({
  draggable: true,
  dragDistance: 5
});

circle.setAttr('dragDistance', 5);
```