output contact_id on /contacts API

picampaign/campaign/serializers.py

@@ -23,7 +23,9 @@ class CampaignContactSerializer(serializers.HyperlinkedModelSerializer):
     last_name = serializers.Field(source='contact.last_name')
     phone = serializers.Field(source='contact.phone')
     groups = serializers.Field(source='all_groups')
+    contact_id = serializers.Field(source='contact.id')
 
     class Meta:
         model = CampaignContact
-        fields = ('id', 'weight', 'first_name', 'last_name', 'phone', 'groups')
+        fields = ('id', 'weight', 'contact_id',
+                  'first_name', 'last_name', 'phone', 'groups')