Adds BLANK_CHOICE_DASH as a choice item

If the model's field is a choice field and isn't required, adds `django.db.models.fields.BLANK_CHOICE_DASH` into the choices. Includes tests.

![screen shot 2013-05-18 at 12 47 09](https://f.cloud.github.com/assets/168319/521463/5a61d1da-bfa8-11e2-81c0-2e167539fe6d.png)