Changes:
* zip_packer can create empty zip archives
* new option to ignore problematic files when creating archive
* detection of non-existent files
* debugging messages for opening of faulty zip archives
* coding style improvements
* no PHP 5.2 hacks
* more unit tests
This replaces the previously defined external_file_exception that was
not used anywhere in the code. The new exception will be thrown in
situations where there is some unexpected problem detected with the
records in the {files_reference} table (e.g. unable to find a record
referenced by some file from {file}, invalid reference value etc.)