SMW\Store\Maintenance\DataRebuilder

After I saw #243, I couldn't take any longer. This PR makes rebuildData a first class citizen. Functionality remains that of SMW_refreshData less the spaghetti.

`php rebuildData.php --d 50 -v`

- Use of the Maintenance class
- Environment setup in done by `rebuildData.php` (represented by `SMW\Maintenance\RebuildData`)
- Execution is done by SMW\Store\Maintenance\DataRebuilder
- Needs some tests