Feature/removal

Adds ability to remove hoverintent behaviour. 

e.g.

```js
var hoverTwo = hoverintent(second, function(e) {
    this.className = 'on';
  }, function(e) {
    this.className = 'off';
  });

// removes hoverintent behaviour
hoverTwo.remove()
```
Fixes https://github.com/tristen/hoverintent/issues/2.