AttributeError: 'ModelOptions' object has no attribute 'get_sorted_fields'
I'd look into updating the code to use `.sorted_fields`. there is also `.sorted_field_names`. This is a performance improvement, as that code had potential to be called a _lot_.
 @rudyryk has a good suggestion here for a temporary solution: https://github.com/flask-admin/flask-admin/issues/1117

```
    for n, f in zip(model._meta.sorted_field_names,
                    model._meta.sorted_fields):
```
 Folks, I'm still getting this error.

Traceback:
```
 File "app.py", line 30, in <module>
    admin.add_view(ModelView(database.Comic))
  File "/usr/local/lib/python3.5/dist-packages/flask_admin/contrib/peewee/view.py", line 144, in __init__
    menu_icon_value=menu_icon_value)
  File "/usr/local/lib/python3.5/dist-packages/flask_admin/model/base.py", line 584, in __init__
    self._refresh_cache()
  File "/usr/local/lib/python3.5/dist-packages/flask_admin/model/base.py", line 653, in _refresh_cache
    self._list_columns = self.get_list_columns()
  File "/usr/local/lib/python3.5/dist-packages/flask_admin/model/base.py", line 729, in get_list_columns
    columns = self.scaffold_list_columns()
  File "/usr/local/lib/python3.5/dist-packages/flask_admin/contrib/peewee/view.py", line 163, in scaffold_list_columns
    for n, f in self._get_model_fields():
  File "/usr/local/lib/python3.5/dist-packages/flask_admin/contrib/peewee/view.py", line 152, in _get_model_fields
    return model._meta.get_sorted_fields()
AttributeError: 'ModelOptions' object has no attribute 'get_sorted_fields'
```

Versions:
```
Flask==0.12.2
Flask-Admin==1.1.0
peewee==2.10.1
wtf-peewee==0.2.6
```

What is going on?



I noticed `get_sorted_fields()`, `get_field_names()`, and `get_fields()` were removed by these commits: https://github.com/coleifer/peewee/commit/2619fcc12b01e34933861de51918507d118655c4 and https://github.com/coleifer/peewee/commit/50c9c1dd24b7a4c48a396a9f5e88f3ea1647859a

It's causing some flask-admin tests to fail, with this error:

```
AttributeError: 'ModelOptions' object has no attribute 'get_sorted_fields'
```

What's the most backwards compatible replacement for get_sorted_fields()? model._meta.sorted_fields?
