i18n+fix: Simplified Chinese translation

[SuperMarioSF]

I have done a translation for Simplified Chinese language and ready for contribute that.

Also, there is a small fix for a translation text missing problem.

models.py line 72

diff --git a/oidc_provider/models.py b/oidc_provider/models.py
index 6504238..2a16c22 100644
--- a/oidc_provider/models.py
+++ b/oidc_provider/models.py
@@ -69,7 +69,7 @@ class Client(models.Model):
                     u' of their credentials. <b>Public</b> clients are incapable.'))
     client_id = models.CharField(max_length=255, unique=True, verbose_name=_(u'Client ID'))
     client_secret = models.CharField(max_length=255, blank=True, verbose_name=_(u'Client SECRET'))
-    response_types = models.ManyToManyField(ResponseType)
+    response_types = models.ManyToManyField(ResponseType, verbose_name=_("Response Type"))
     jwt_alg = models.CharField(
         max_length=10,
         choices=JWT_ALGS,

The translation is missing for "Response Type". This also affected other languages' translation.