PLXCOMP-220: added unit tests for handling invalid uid/gid in TarEntry

Added extra unit tests to verify that invalid uid/gid values never make it into Tar output, and are replaced with zeroes.

In addition, I ran a quick jython test (python tarfile module could not read tar files produced by plexus before):

    Jython 2.5.3 (2.5:c56500f08d34+, Aug 13 2012, 14:48:36)
    [Java HotSpot(TM) Client VM (Sun Microsystems Inc.)] on java1.6.0_25
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import tarfile
    >>> f = tarfile.open("D:/projects/plexus-archiver/target/output/archive.tar")
    >>> f.list()
    -rw-r--r-- 0/0         25 2013-01-03 22:04:59 TarGZipUnArchiverTest1.txt
    -rw-rw-r-- 0/0         62 2013-01-03 22:04:59 TarGZipUnArchiverTest2.txt