GreenInvitation: add send_reinvite() method

darcs-hash:20090423090236-b2873-f4476b840d4c791736654aa200dfcb3449eece8d
GreenInvitation: rename 'connected' property to 'confirmed' to be in line with the state

darcs-hash:20090423090335-b2873-d863f54c422c51c51f26d94973cbc524a4dd89b7