Hiding position row

Its PR for issue https://github.com/sonata-project/SonataDoctrineORMAdminBundle/pull/102#issuecomment-27499091

before this patch
![beforepatch](https://f.cloud.github.com/assets/981783/1715444/1ca734f4-61b3-11e3-8fde-7a9856213766.png)

after this patch
![afterpatch](https://f.cloud.github.com/assets/981783/1715445/221a9c32-61b3-11e3-8184-ddf1dead03fe.png)

Sorting functionality works ok.
Maybe new style should be added to the sonata admin bundle - I can do it, if its needed