fix pencil not working when used in django.admin.*Inline

because the latter is generating classes with hyphens ("id_model_set-n-field", where "n" is integer of inline, starting from 0)
![image](https://f.cloud.github.com/assets/2611009/585153/83b565c4-c930-11e2-8b01-cd5fd63c3cf7.png)