Populate values for ApplicationContent admin_fields from parameters

When adding a field to ApplicationContent as described in the [docs](http://feincms-django-cms.readthedocs.org/en/latest/integration.html#additional-customization-possibilities), the field value will never show up correctly in the admin form.

Assuming something like this:
```
Page.create_content_type(ApplicationContent, APPLICATIONS=(
  ('registration', 'Account creation and management', {
      'urls': 'yourapp.registration_urls',
      'admin_fields': {'greeting': forms.CharField()},
  }),
)
```

The ApplicationContent for registration will show an additional greeting field once it's saved. When a greeting is entered into the field and the page gets saved, the value will be stored correctly in the database. However, the greeting will be empty on reloading the admin page.

I've tried to come up with a simple fix for this including a test. I'm not sure if squeezing the value into initial is a good idea though. What do you think?