Update jquery.sticky.js

#Adds some new functionality: 

## Remove stickiness

For example, on a progressive layout some elements might be sticky on a large screen but made unsticky if the window is made smaller:

```{.js}
$('foo').sticky('unstick');
```

## Reapply stickiness

If the sticky element's size changes, for example because a menu opened, allows the stickiness to be laid out again.

```{.js}
$('foo').sticky('unstick', options);
```