Adapted session.py to new Invitation interface (untested)

darcs-hash:20090127171855-58103-50e83210284e4d639a9e5fc82581a1acd11a76b9
Added changes requested by Willem Toorop regarding reading commandline options for MSRP scripts

darcs-hash:20090127172230-58103-5ed804ce0601f363d6c4528057501ab421332a9d