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 the Maintenance class
- Environment setup in done by SMW\Maintenance\RebuildData
- Execution is carry out by SMW\Store\Maintenance\DataRebuilder

- Needs some tests