Updated Jxl api to 2.5.7

The JXL api (Excel workbook reading and writing) has been updated to the last version : 2.5.7.

Version 2.5.7 (July 2005)

Handled variable arg functions which have zero parameters, such as row() and column()
Fixed WritableHyperlink so that it would handle file links (in particular file links located on a different drive letter)
Made XFRecord handle uninitialized records, and enhanced the equals() method for Font and Format to allow better copying between sheets
Detected a corrupt biff file format to throw an exception, rather than hanging in an endless loop
When writing a sheet, added to check to stop the addition of a null WorkspaceOptions object
Ignored the type object type MSOFFICDRAWING (as distinct from PICTURE) when reading/copying spreadsheets with pictures in them
Ignored, and warned, when processing charts on non biff8 workbooks
Display a warning when standard property sets aren't present in the original sheet
Display a warning about spurious block chains the small block stream
Fixed a bug when copying dates which consist of just a time - one day was subtracted from the value
Applied contributed fix for copying cell borders with colours
Added the ability to remove a comment from a cell
Applied Adam's patch to add support for horizontal/vertical centring when printing