Made Engine thread non-deamonic to prevent possible crashes

See section 17.2.9 of the Python library documentation.

WARNING: This patch means that whenever an application starts the Engine,
         it is also responsible for stopping it, whatever happens.

darcs-hash:20090428094917-58103-cc354a1e0ae2614a6de2611ed8c3cd92266d0e14
fix bug in GreenInvitation.respond_to_invite: it used to hang up because it expected GotSDPUpdate notification

darcs-hash:20090428100348-b2873-116e98d035634179a6185982b488b75a5bc52a74