Enable dragging across image borders

Currently the drag event is cancelled as soon as the user drags the crop area outside of the cropper container. I implemented a way to enable dragging across the image borders, so the event doesn't get cancelled.

Changes in detail:

- A new property `isDragging` which indicates if the user is currently dragging or not.
- Setting/checking that property in the `dragstart`, `dragmove` and `dragend` functions.
- `event.preventDefault()` will be executed every time in `dragmove`, not only when `touching` is true. This ensures that no text will be selected, when the user drags outside of the cropper container.
- Little typo fixing.

As described in #54 this needs some fixing, but hopefully is a solid base.