Support passing methods to list_display

Imagine having a simple MongoEngine document and a model admin:

```
class User(Document):
    email = StringField(required=True)
    first_name = StringField(max_length=50)
    last_name = StringField(max_length=50)

    def get_full_name(self):
        return '%s %s' % (self.first_name, self.last_name)

class TestAdmin(model.ModelAdmin):
    list_display = ['get_full_name']
```

It would show `<bound method Test.get_full_name of <Test: Test object>>` in the list. This pull request fixes it by checking if an attribute is callable.