this.tags is on the prototype instead of the element.

This:

``` javascript
Tagging.prototype = {
        tags: [],
...
```

Should be:

``` javascript
var Tagging = function(elem, options) {
        this.elem = elem;
        this.$elem = $(elem);
        this.options = options;
        this.tags = [];
}
```

In order to allow a user to use more than one tagging block per page.
