fix GreenInvitation and GreenRegistration to raise errors instead of returning raw notification

add protection to GreenRegistration methods from being called in invalid state / more than once

darcs-hash:20090211132243-b2873-5a68f338158d792559c5c0071868b7dd10f51745
Update TODO

darcs-hash:20090211135707-58252-dcfe2183f128881b02042fed9e0fe6a126b76d1a