Self-registration of new sites

[Hendrik Mans]

1. When a new #pants instance is set up, you should be able to set up your site through the web (instead of running a command through the Rails console.)
2. You should be able to set up your instance to host other sites by showing a "claim" form when no site/user was found for the requested host.
3. It should be possible to configure this self-registration to require a token generated by the instance's owner (or be free-for-all.)