Stop defaults from being overwritten

Commit message:
>## Stop `$.extend` overwriting `defaults` object.

>When using `$.extend`, the first argument object is altered, and
since objects are copied by reference in Javascript that means the
`defaults` object gets changed when it shouldn't and subsequent
calls to `$().sticky()` will use previous options rather than fresh
defaults.

More info:
```js
var defaults = {
  // defaults set in jquery.sticky.js
  topSpacing: 0,
  bottomSpacing: 0,
  className: 'is-sticky',
  wrapperClassName: 'sticky-wrapper',
  center: false,
  getWidthFrom: ''
};
console.log(defaults.center);
// false
var options = {
  // options passed in to $(element).sticky()
  center: true
};
$.extend(defaults, options)
console.log(defaults.center);
// true
// Subsequent sticky calls will be centered.
```