Add "origin" field to the Pin model

"origin" is an optional field that stores the URI for the site
that the image has been saved from, it's going to be used only from
bookmarklet. Fixes #63

pinry/core/api.py

@@ -114,7 +114,7 @@ class PinResource(ModelResource):
         return super(PinResource, self).save_m2m(bundle)
 
     class Meta:
-        fields = ['id', 'url', 'description']
+        fields = ['id', 'url', 'origin', 'description']
         ordering = ['id']
         queryset = Pin.objects.all()
         resource_name = 'pin'