Updated Tagging entity

Fixes invalid entity

``` The field Kunstmaan\TaggingBundle\Entity\Tag#tagging is on the inverse side of a bi-directional relationship, but the specified mappedBy association on the target-entity Kunstmaan\TaggingBundle\Entity\Tagging#tag does not contain the required 'inversedBy=tagging' attribute.
Merge pull request #3 from kolah/patch-1

Thanks!