Make batch modal overflow visible

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32510&start=0

When we using batch modal, the latest chosen selector will be hidden by modal frame.

![p2013-10-29-1](https://f.cloud.github.com/assets/1639206/1459095/b42d107a-43c0-11e3-80e2-dc3ef8827012.jpg)

I added a class `.overflow-visible` in isis. If we add this class to `modal-body`, the latest chosen will not been hidden anymore.

``` html
<div class="modal-body overflow-visible">
```

![p2013-10-29-2](https://f.cloud.github.com/assets/1639206/1459106/9ba33b46-43c1-11e3-8688-b5b149071841.jpg)


And if we have to show a long content in modal, just remove this class:

``` html
<div class="modal-body">
```

We can see the scroll bar appeared.

![p2013-11-02-1](https://f.cloud.github.com/assets/1639206/1459107/a0b04656-43c1-11e3-84b4-c42a031f6e8d.jpg)