Added a new method GetSortedColumnsExpression() to ColumnCollection

This new method uses the result of GetFilteredColumns() and returns is as an expression. e.g. "columnname asc, othercolumn desc"

The returned expression can be used with the OrderBy(string sortExpression) extension mehod
found here : http://extensionmethod.net/csharp/ienumerable-t/orderby-string-sortexpression