Little changes to avoid bad style to underlying html elements

Little modifications to remove strange effect to underlying html elements in django admin

You can see what happens before 
![schermata 2014-11-14 alle 12 53 38](https://cloud.githubusercontent.com/assets/2088831/5052204/9e4c322a-6c3e-11e4-8554-e1d23c253258.png)

and after my modifications
![schermata 2014-11-14 alle 12 54 10](https://cloud.githubusercontent.com/assets/2088831/5052207/aa485496-6c3e-11e4-91e0-0d642e270371.png)