Fixed _pjsip_msg_to_dict function

Ignore-this: 66a2d7510ca7b687986b7b31a8bb28f9

darcs-hash:20090708073658-1e3d8-8dcde54d9bf560289d1b2daee83c472d865a4ebc
Refactored Invitation class

Ignore-this: 5b7b4e856a89541312ad17735092e70c

darcs-hash:20090708132825-1e3d8-512513bd40584de8ec4f5333c253a5728cce4dd6