Minor method name/responsibilty improvement

When reading the code I found some of the names too generic to follow. I've done the following renaming:
- define_fields -> define_db_field
- define_fields_method -> define_fields_whitelist
- fields_method_name -> fields_whitelist_name

I've also moved "define_fields_method" out of "define_fields", since the two serve unique responsibilities. So in the attach method your now have:

```
ToCallback#attach
      define_fields_whitelist
      #...  define_callback, etc

FromCallback#attach
      define_db_field
      define_fields_whitelist
      #...  define_callback, etc
```

If you don't like my names, a simple find/replace can change them to something else...

This PR includes my previous mongoization PR as well (see #21)