Add the Response model

This is to store RSVPs.

calsocial/models.py

@@ -103,6 +103,24 @@ class ResponseType(Enum):
         return Enum.__eq__(self, other)
 
 
+class EventAvailability(Enum):
+    free = 0
+    busy = 1
+
+
+class UserAvailability(EventAvailability):
+    tentative = 2
+
+
+class ResponseVisibility(Enum):
+    private = 0
+    organisers = 1
+    attendees = 2
+    followers = 3
+    friends = 4
+    public = 5
+
+
 class SettingsProxy:
     """Proxy object to get settings for a user
     """