Unless there is a real use for them, I've been leaving off the URL properties that point to API resources on all the other types, since they are really only useful when truly traversing API resources in a RESTful fashion.  This library doesn't really do that.  ([see previous discussion here](https://github.com/sqs/go-github/commit/515d5d730cc7d039ec7855bccb38940f67756c23#commitcomment-3381346))