Now it is possible to specify a getter in 'field_name' on relationship

When 'options_sort_field' option is specified only (for backward
compability).  If it isn't, 'name_field' will be used as column name for
sorting.
Some fixes after code review
Merge pull request #218 from neoascetic/bugfix/fieldname_specify_getter_on_relationship

Now it is possible to specify a getter in 'field_name' on relationship