* Added the listFields() method to PlaceDataSource and subclasses. FOr most this is a stub, but for ArcGIS REST this is working.

* Admin panel for PlaceDataSource items, now uses the 'isfield' setting for $option_fields, generating a list of fields for selection.

* To the FAQ added new question "why is the ___ place/event source not supported" and added the WritingDrivers.txt document describing how one would go about that.
* Admin UI: the _place_source_ method now only fetches field names if it really needs them for the form, e.g. if an option in fact has 'isfield' set

* Place Data Source, ArcGIS REST driver: selected name field and description field (option1 and option2) are now checked against the remote list of field names, ensuring that they in fact exist