First cut of the unregister function

This adds a new feature to the server allowing people to unregister packages.

You send an HTTP `DELETE` request to `https://{server url}/packages/{package name}?access_token={token}` where `token` is an OAuth token issued by GitHub.

The request will succeed if and only if you have commit access to the github repo that is registered for the package.

Actually generating the tokens for a user will be handled in an update to the client.