Missing an `admin_invite_user` view

A view is missing in order to invite users using the signup-code feature available.
In the original project from pinax it seems that it was named [`admin_invite_user`](https://gitorious.org/pinax/pinax/source/7c09913e46d3c44c4693271a4b6c6b4cb4f01acb:pinax/apps/signup_codes/views.py#L100).

I'm planning on adding it, making this issue a PR, don't hesitate to stop me if there is a good reason or that it's on purpose that there is no such view available.

Cheers