Autoscroll when dragging

Autoscrolls the container element while dragging if you drag the element over the offsets defined in scrollTriggers config option.

Also you can enable/disable this behaviour (disabled by default to be backwards compatible) and control the scroll sensitivity and speed.

New available options are:
```
* scroll                  (false)   Enable or disable the scrolling behaviour
* scrollSensitivity       (1)       Mouse movement needed to trigger the scroll
* scrollSpeed             (5)       Speed of the scroll
* scrollTriggers          (obj)     Distance from the border where scrolling become active
    top: 40
    left: 40
    right: -40
    bottom: -40
```