Merge cells for worksheet headers and footers

Files:
modified: /trunk/src/excel/org/jboss/seam/excel/ExcelWorkbook.java (try)
modified: /trunk/src/excel/org/jboss/seam/excel/csv/CsvExcelWorkbook.java (try)
modified: /trunk/src/excel/org/jboss/seam/excel/ui/UIWorksheet.java (try)
modified: /trunk/src/excel/org/jboss/seam/excel/jxl/JXLExcelWorkbook.java (try)