Next and prev functions should be optional

@stewartyu @tedtate 

Basic loop usage:

```js
        $carousel.scooch({
            next: function() {
                var index = this._index + 1;

                if(index > this._length) {
                    index = 1;
                }

                this.move(index);
            },
            prev: function() {
                var index = this._index - 1;

                if(index == 0) {
                    index = this._length;
                }

                this.move(index);
            }
        });
```

Basic jumping:

```js
        $carousel.scooch({
            next: function() {
                this.move(this._index + 2);
            },
            prev: function() {
                this.move(this._index - 2);
            }
        });
```