Update tarfile summation to handle undecodable unicode errors

This patch fixes the following exception:

Exception on /v1/images/3072260786c47aa6dd3ae39d860f1423ace97f42b431a9f403da01206d75f1a1/layer [PUT]
Traceback (most recent call last):
  File "/home/dotcloud/env/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/dotcloud/env/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/dotcloud/env/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/dotcloud/env/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/dotcloud/registry/toolkit.py", line 203, in wrapper
    return f(*args, **kwargs)
  File "/home/dotcloud/registry/images.py", line 159, in put_image_layer
    for member in tar:
  File "/usr/lib/python2.7/tarfile.py", line 2470, in next
    tarinfo = self.tarfile.next()
  File "/usr/lib/python2.7/tarfile.py", line 2323, in next
    tarinfo = self.tarinfo.fromtarfile(self)
  File "/usr/lib/python2.7/tarfile.py", line 1240, in fromtarfile
    return obj._proc_member(tarfile)
  File "/usr/lib/python2.7/tarfile.py", line 1262, in _proc_member
    return self._proc_pax(tarfile)
  File "/usr/lib/python2.7/tarfile.py", line 1392, in _proc_pax
    value = value.decode("utf8")
  File "/home/dotcloud/env/lib/python2.7/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0xdb in position 16: invalid continuation byte