decode_auth function does not handle utf-8 logins or password
HI

I have found that the function **decode_auth** (line 96, [file](https://github.com/docker/docker-py/blob/master/docker/auth/auth.py)) fails when decoding UTF-8 passwords from the .dockercfg file, and **load_config** returning an empty config.

I have checked and docker hub can handle UTF-8 passwords, this code proves that:

``` python
# coding=utf-8
from docker import Client
cred = { 'username': <user>, 'password': <utf-8 password> }
c = Client(base_url='unix://var/run/docker.sock')
res = c.pull(repository='<private container>', tag='latest', auth_config=cred)
print(res)
```

Thank you

diff --git a/docker/auth/auth.py b/docker/auth/auth.py
index 2ed894ee..416dd7c4 100644
--- a/docker/auth/auth.py
+++ b/docker/auth/auth.py
@@ -96,7 +96,7 @@ def decode_auth(auth):
         auth = auth.encode('ascii')
     s = base64.b64decode(auth)
     login, pwd = s.split(b':', 1)
-    return login.decode('ascii'), pwd.decode('ascii')
+    return login.decode('utf8'), pwd.decode('utf8')
 
 
 def encode_header(auth):
diff --git a/tests/unit/auth_test.py b/tests/unit/auth_test.py
index 9f4d439b..67830381 100644
--- a/tests/unit/auth_test.py
+++ b/tests/unit/auth_test.py
@@ -316,3 +316,33 @@ class LoadConfigTest(base.Cleanup, base.BaseTestCase):
             self.assertEqual(cfg['password'], 'izayoi')
             self.assertEqual(cfg['email'], 'sakuya@scarlet.net')
             self.assertEqual(cfg.get('auth'), None)
+
+    def test_load_config_custom_config_env_utf8(self):
+        folder = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, folder)
+
+        dockercfg_path = os.path.join(folder, 'config.json')
+        registry = 'https://your.private.registry.io'
+        auth_ = base64.b64encode(
+            b'sakuya\\xc3\\xa6:izayoi\\xc3\\xa6').decode('ascii')
+        config = {
+            'auths': {
+                registry: {
+                    'auth': '{0}'.format(auth_),
+                    'email': 'sakuya@scarlet.net'
+                }
+            }
+        }
+
+        with open(dockercfg_path, 'w') as f:
+            json.dump(config, f)
+
+        with mock.patch.dict(os.environ, {'DOCKER_CONFIG': folder}):
+            cfg = auth.load_config(None)
+            assert registry in cfg
+            self.assertNotEqual(cfg[registry], None)
+            cfg = cfg[registry]
+            self.assertEqual(cfg['username'], b'sakuya\\xc3\\xa6'.decode('utf8'))
+            self.assertEqual(cfg['password'], b'izayoi\\xc3\\xa6'.decode('utf8'))
+            self.assertEqual(cfg['email'], 'sakuya@scarlet.net')
+            self.assertEqual(cfg.get('auth'), None)
['tests/unit/auth_test.py', 'docker/auth/auth.py']