Changed serializer plugin to use PHP's builtin serialize() function instead of using XML_Serializer.

Fixed bug introduced in last commit; ported bagit_build_file_extra() from D6; updated bagit_plugin_copy_fileimage.inc.

Issue #1449890: Replaced test fetch plugin in D7 version with one that adds values from the contrib Link module.

Added check to hook_file_download() in D7 branch to make sure the file being downloaded is controlled by this module.

Added support for fetch.txt plugins to D7 branch, plus a sample plugin that will be replaced later by one based on the Link contrib module.

Moved function to get fields of file or image type from plugin to main D7 module so other plugins can use it.