Separate wrapper functions into CKAN core and extra

[inghamn]

In order to make it easier for agencies to adapt their own fields, it might be nice to split up the wrapper functions.

Once we start reading from CKAN core fields as much as possible for POD fields, it might make sense to have core CKAN field wrappers and agency-defined wrappers for any extra fields they define. It might be worth a little bit of reorganizing class files in the source code to make it more obvious where to go to extend the core extension fields.