Add support for multiformfield, fix instance logic, generalize field shortcut

  A multiformfield is a field that is composed of a number of
  subfields.  You can access the subfields individually, as for a
  database, or as a unit by addressing the multiformfield (which
  concatenates the subfield values together).

  Added PostalCodeFormField and CountryFormField.

  field, optField now take options for passing limits to
  NumberFormFields and choices to ChoiceFormFields.

  Added Form::SQLTable which can be used to derive the corresponding
  SQL table that could back the form.

  Fixed multiple instances to not use [] post syntax, since that seems
  not to preserve order.  Better to just give each instance its own
  post var.