Modified topOffset option and added $.scrollIt.destroy() function to remove event handlers

Breaking Change:
Modified the topOffset option to accept an object with { val : 0 }. The way Javascript works, this enables us to keep a variable outside of scrollIt like this:

```javascript
var topOffset = {
  val: -30
}

$.scrollIt({
  topOffset : topOffset
});

topOffset.val = -90; // scrollIt will now use updated topOffset
```

Second change: I added a function to remove the scrollIt event handlers. This is quite useful when building a single page app or using frameworks like Ember, so that you can cleanly remove scrollIt and re-initialize it when you move around the page.

Use ` $.scrollIt.destroy()`

Cheers!