GPG signature support on commit object.

Current version fails if any commit object has GPG signeture.

It works by just deleting lines from 433 to 440 of git/objects/commit.py.

I can't find documentation of git object format. But I tried to write support
GPG signature on commit object.

Please consider to merge.

Here is the way to reproduce problem:

    sugi@vaj-k-334-sugi:~% mkdir signed-test
    sugi@vaj-k-334-sugi:~% cd  signed-test
    sugi@vaj-k-334-sugi:~/signed-tes/% git init
    Initialized empty Git repository in /home/sugi/signed-test/.git/
    git@vaj-k-334-sugi:~/signed-test% date > date.txt
    sugi@vaj-k-334-sugi:~/signed-test% git add .
    sugi@vaj-k-334-sugi:~/signed-test% git commit -S -m 'Test commit with GPG sig.'
    
    You need a passphrase to unlock the secret key for
    user: "Tatsuki Sugiura <sugi@nemui.org>"
    1024-bit DSA key, ID 4502FDC2, created 2000-09-30
    
    gpg: gpg-agent is not available in this session
    [master (root-commit) afad9ad] Test commit with GPG sig.
     1 file changed, 1 insertion(+)
     create mode 100644 date.txt
    sugi@vaj-k-334-sugi:~/signed-test% git cat-file commit HEAD
    tree 9165a13698f6b211db3d9631958ef11e8d06d477
    author Tatsuki Sugiura <sugi@nemui.org> 1336266773 +0900
    committer Tatsuki Sugiura <sugi@nemui.org> 1336266773 +0900
    gpgsig -----BEGIN PGP SIGNATURE-----
     Version: GnuPG v1.4.12 (GNU/Linux)
     
     iEYEABECAAYFAk+l0BUACgkQbK2FzkUC/cLR9wCeKUkjeA39+vy0T/TwJuqPhca3
     eNMAnRw0tDNAqIcM4HJnHvzCsu86dLGV
     =ZlAu
     -----END PGP SIGNATURE-----
    
    Test commit with GPG sig.
    sugi@vaj-k-334-sugi:~/signed-test% python    
    Python 2.7.3rc2 (default, Apr 22 2012, 22:35:38) 
    [GCC 4.6.3] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import git
    >>> git = git.Repo("/home/sugi/signed-test")
    >>> git.commit()
    <git.Commit "afad9ad7986c98784288ae71deb696d505cdfe5c">
    >>> git.commit().author
    afad9ad7986c98784288ae71deb696d505cdfe5c
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/home/sugi/lib/python2.7/site-packages/gitdb-0.5.4-py2.7-linux-x86_64.egg/gitdb/util.py", line 238, in __getattr__
        self._set_cache_(attr)
      File "/home/sugi/lib/python2.7/site-packages/GitPython-0.3.2.RC1-py2.7.egg/git/objects/commit.py", line 133, in _set_cache_
        self._deserialize(StringIO(stream.read()))
      File "/home/sugi/lib/python2.7/site-packages/GitPython-0.3.2.RC1-py2.7.egg/git/objects/commit.py", line 444, in _deserialize
        self.author.name = self.author.name.decode(self.encoding) 
    LookupError: unknown encoding: -----BEGIN PGP SIGNATURE-----