$.plax.disable() with non-default activity target 

The disable function seems to use the window as the mouse movement listener. I have changed this to:

```
disable: function(opts){
      plaxActivityTarget.unbind('mousemove.plax');
      ...
```

This seems to work with my setup using the following options.  

```
$.plax.enable(
    { "activityTarget" : $('.phonecontainer') }
);
```
