add client credentials grant

Author: Kevin Hannegan

presalytics/client/oidc.py

@@ -252,16 +252,18 @@ def _handle_response(self, response):
                     pass
         return data
 
-    def client_credentials_token(self, scope=None):
+    def client_credentials_token(self, audience=None, scope=None):
         if not self.client_secret:
             raise presalytics.lib.exceptions.ApiError(message="Must have client secret for client credentials grant", status_code=400)
+        if not audience:
+            audience = self.audience
         if not scope:
             scope = self.default_scopes
         post_data = {
             "client_id": self.client_id,
             "client_secret": self.client_secret,
             "grant_type": "client_credentials",
-            "audience": self.audience
+            "audience": audience
         }
         repsonse = self._post(self.token_endpoint, data)