Fix category batch copy

### Issue
Try batch copy a category if
* the category itself has subcategories
* you also change the language or access level with the same batch process

This will fail with a fatal error in case of batchAccess or a lengthy error message in case of batchLanguage. The categories will be copied (sometimes even multiple times), but the access levels and languages are not correctly changed.

### Bugtracking
The problem is that the table->reset() function didn't reset the `_location` and `_location_id` properties of the table. Thus subsequent stores tried to move the categories again, which failed due to wrong parent ids.

### Fix
This PR adds JTableNested->reset() which will reset the locations (`using $this->setLocation(0)`) after the parent reset function was executed.

Fixes https://github.com/joomla/joomla-cms/pull/2628
Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32827