Use class attribute to guide RDN creation

[SwampFalc]

So in #175 it was mentioned that, ideally, the whole process of creating the RDN should be separate from designating primary keys for the database table.

@Natureshadow mentioned they were not keen on such a major rewrite, though.

But looking at the code, I believe there's quite an easy path, namely a simple class attribute, for which I propose rdn_fields, taking a list of field names. The code for build_rdn is trivial to change.

As far as the user is concerned:

1. They are already expected to add some class attributes. Requiring one more should not be strange or surprising.
2. Previously, if they did not add a primary_key field, they got an error when saving. Now, if they do not add the class attribute, they get the exact same error at the exact same time.
3. Creating a list of field names is a common occurrence in Django: for Meta ordering, for the admin site, for a ModelForm, etc.

In short, as far as user experience goes, I do not see this as a major difference.

Is it backwards compatible? Well, not as I wrote it, no. But you can easily turn the if slightly more complicated to achieve that:
if field.db_column and (field.primary_key or field.name in self.rdn_fields):

[Natureshadow]

@Natureshadow mentioned they were not keen on such a major rewrite, though.

Where did I say that?

In any case, please make this change backwards-compatible.

[SwampFalc]

I was referring to this comment: #175 (comment)

[Natureshadow]

I was referring to this comment: #175 (comment)

I can't remember what the reason for this comment was. That said, I will test and review the change in the course of the week.