Modified the XLConnect jar to use "cell.getDateCellValue()" instead of t...

Modified the XLConnect jar to use "cell.getDateCellValue()" instead of the getJavaDate method. The old method assumed that the worksheet was using 1900 windowing, so dates from sheets with 1904 windowing were off by four years and a day. The method getDateCellValue automatically adjusts the date depending on the windowing used by the sheet.

This pull request will be accompanied by a request in the xlconnect-java repository, with the actual source code.