modified core.js to allow the user to uniquely identify column's defined with function datasources

Note: For this to ensure uniqueness the user must modify the accessor factory function so that the accessor returns a string to uniquely identify the column.

Something like so:

        function property(attr,columnName) {
          columnName = typeof columnName === 'undefined' ? attr : columnName;

          return function (row, value) {
            if (typeof row === 'undefined' ) return columnName; 
            return row.attr(attr, value);
          }
        }