optional property to dynamic settings : order on group

The origin:
--------------
I create a django app with a lot of dynamic settings.
I want to find it in the same group.
But, also order this var on group...

Before my patch, i do this:
--------------------------------------
```python
register_setting(
    name="APPNAME_1"
)
register_setting(
    name="APPNAME_2"
)
```
After:
--------
To reuse this name on template, views etc. it's very confusing...

I add a optionnal "order" properties on register_setting like this :
```python
register_setting(
    name="APPNAME_ADDRESS",
    order=0
)
register_setting(
    name="APPNAME_ZIPCODE",
    order=1
)
```

On a long propertie list, you can do :
```python
def autoIncrement():
    i = 0
    while 1:
        yield i
        i += 1

inc = autoIncrement()


def order():
    return inc.next()

register_setting(
    name="APPNAME_ADDRESS",
    order=order()
)
register_setting(
    name="APPNAME_ZIPCODE",
    order=order()
)
.
.
.
```