bugfix for dynamic fk add (new items were not deselectable)

When in the admin interface I add an other foreign key, I can see the new item selected. Unfortunately it was unselectable if new. Now it is selectable.
To better express the concept, see the attachments..

![schermata 2014-11-25 alle 15 06 01](https://cloud.githubusercontent.com/assets/2088831/5184159/3d782c36-74b5-11e4-8698-3c5250fed152.png)

![schermata 2014-11-25 alle 15 06 46](https://cloud.githubusercontent.com/assets/2088831/5184158/3d5f8c58-74b5-11e4-9e2a-4568248996a2.png)

N.B. I used jquery "on" method, that is supported only from version Django 1.5